PMCs are not necessarily a bad investment. Almost nothing is a automatically a bad investment. It all depends on the price they're being sold for.
If you look at the currently marketed PMCs, then most/all of them will not give you back your investment at their current price unless the difficulty rise slows down soon. IPO prices for these assets were even higher.
Why are current prices so high? People still "believe" in PMCs because they haven't done the math or think that difficulty growth will slow down very soon. So there is buying pressure to slow down the price decrease.
Why were IPO prices so high? Ask the people issuing these assets.
So your answer is that people are bad at math or that issuers are pricing their value add too high?
Basically, yes.
Most PMCs don't have any reinvestment plan, which means that as the difficulty increases, the asset decreases in price alongside its dividends. I think that many people who buy PMCs don't realize how rapidly the asset-price can decrease. I guess that everyone realizes that dividends will go down with difficulty, but there are people who expect that they're able to resell the asset for a similar price to what they paid for it after some time (as evidenced by a discussion from the TAT.VIRTUALMINE thread a while back, where someone was upset that the asset-price had gone down and demanded a refund).
On a different note, anyone interested in PMCs and their price-development, should look at the DMS set of assets and its price/dividend developments.
Reinvestment is a huge red herring - if something is making a loss in the first place then all reinvestment does is compound that loss.
The fundamental error OP is making is working on the assumption that there IS a way to profitably invest in mining. In general mining is, at best, marginally profitable (exceptions being those that get in EARLY on new technology - early has now passed for ASICs) - that leaves very little scope for a manager to take a fee and investors to still make a profit.
The basic theory is simple : all the time mining is profitable more people will mine until it ceases to be profitable. That then gets compounded because when BTC rises vs USD mining becomes profitable (in USD terms) again - so mining capacity again increases. That not only removes the profit from new miners but guarantees a loss (in BTC terms) for existing miners/investors - whilst they end up making a USD profit (sometimes) they're inevitably going to be worse off than if they'd just held BTC throughout.
If you believe BTC will rise vs USD there's NO long-term profitable way to invest in mining (by which I mean ending up with more than if you'd just held BTC). All the companies doing reinvestment achieve is compounding their losses (in BTC terms) whilst at the same time giving the illusion of making more profit by paying out more dividends whilst their book value falls far faster. And, of course, if BTC doesn't rise vs USD then that means it's stagnating and dieing anyway - as it HAS to rise if demand/use increases (due to supply definitely not increasing much and quite possibly falling in terms of BTC available for circulation/use).
The reason PMBs/PMCs perform worse than mining shares is two-fold:
1. A real reason : the massive markups often placed on their price.
2. An illusory reason : The visibility of it. Even when they aren't performing worse than other mining investments they give the impression of doing so as their value (the dividends they pay) are visibly dropping. In comparison mining companies don't depreciate their hardware or even give book values in BTC - so the (often as bad or worse) losses occurring there are hidden and so don't get reflected in market price. That's until someone notices what the emperor's new clothes actually are - and the share price crashes.
Any mining share valued at above its Book Value is unlikely to ever make a profit if you invest and hold - as the manager's fees will inevitably outweigh any marginal profit made and reinvestment will turn out to be throwing good money after bad. And if BTC rises at all significantly a loss (in BTC terms) is assured.
There's a few simple questions to ask when looking at a mining investment:
1. Will the manager take fees/shares even if investors don't make a profit?
2. Where's the manager's projection of future difficulty - and how has it compared so far to what actually happened?
If they can't answer 2 then they don't even claim to know it'll make a profit - not many businesses get away with raising capital without even a token effort to establish that there'll be a profit. Which explains why the answer to 1. is nearly always "Yes - they get a cut of revenue not profit : turning a small loss for investors into a large one".
There's only a small set of securities that should be allowed to operate without some reasonable effort to determine that investors will make a profit - investments which are by their very nature speculative. Things like:
Precious metals funds - which are explicitly speculation on something outside the control of the issuer (the price of PMs).
Options, Futures and similar - where investors effectively bet with one another on things (DMS is this type of operation - those holding SELLING and MINING are basically betting against one another on what the future dividends from 1 MH/s will be. It's explicit that some of them will win at the expense of others who lose).
Anything which actually plans to operate a business should determine whether it'll make a profit before asking for cash. Mining investments don't do this - with the general excuse being that it's hard to work out how difficulty will change. That's true - so look back rather than forward - and see how horribly mining investments did in the past. Now try to explain why the same won't happen again - why hashing power will stop being added before it becomes unprofitable.
You DON'T need to predict future difficulty to work out that mining investments in general won't make a profit. Common-sense and history tell you that they'll make a loss in BTC terms unless BTC falls vs USD.