Yes, they're a big fat target. (*) But they spend many orders of magnitude more on security than I do.
Breaking into my house is trivial. Breaking into the bank where Coinbase stores their cold wallets is much tougher.
(*) Although, perhaps less of a fat target than you might think, as they surely don't store all their BTC in a single location.
Breaking into your house is trivial, but breaking into a house where there are 20BTC stored in an unencrypted wallet is pretty much astronomical odds..... there'd be less variance for the burgler breaking into 20 houses and stealing diamond rings. As long as you're not advertising your real name and location in conjunction with your BTC address, I see no reason why this is something that one would reasonably fear.
The odds that are important are the odds that someone is going to break into my house and steal my computer.
They're not high, but they're not astronomically low, either.
But while we're on this subject, if I'm going to store BTC at home, it's going to be in a fire safe, not on a computer, anyway. The risk that my house is going to burn down is also not high, but not astronomically low, either.
(The chance that a house where there are 20BTC stored in an unencrypted wallet will burn down.... Well that's irrelevant.)
(If you live in a high-crime area where break-ins commonly occur, and a computer is a likely candidate to be stolen just for its street value, you should make a copy of your encrypted private keys and put the copy somewhere safe, so if your computer is stolen then you will be able to recover them quickly and transfer the BTC to a new address.) Whereas you can bet that there are literally hundreds of hackers (some intelligent, some not so much) trying to crack Coinbase, et al. on a daily basis.
If you're going to do that, why bother keeping them on the computer in the first place?
That said, I'd say it's still more likely someone is going to break into my house and steal my fire safe (pretending for the moment that I have one), than that someone is going to rob the safety deposit boxes where Coinbase stores their wallets.
Well, I guess it comes down to a matter of trust. Trust in yourself and your ability to protect your property, versus trust in a third party such as Coinbase, given that they are a much bigger and public target.
I know that I have my laptop physically with me at almost all times, and it is backed up at least weekly on a hard drive stored in a location under my control. My important keys (BTC mostly) are replicated up a 3rd time on a USB drive. All encrypted, of course. I know, know for a FACT, that if any of these are compromised, I will be aware of it within a short period of time, and be able to move the BTC to another address before the encryption can be cracked. I also know for a FACT, that without one of these 3 being compromised, I will not ever have to worry about my keys being stolen.
With BTC stored with a third party, there are many unknowns, most of which cannot be verified by me. How many employees have access? How many times has the code been audited? Can I read the code myself? Do they make their addresses and balances public? Without a VERY high level of transparency, I can never know for a FACT that the BTC stored with them are safe. Therefore, I must trust them in order to store my BTC with them. And I simply will not choose to do that, when there are simple security measures that I can undertake personally, and which will not restrict my use of my BTC in any way, or require me to undertake anything onerous.