There will always be a disadvantage to everything, and in trading, one of them is that when you do a demo account, you might bring something you are not supposed to bring in an actual account. I mean, you are right, those who practice and become familiar with demo accounts should adjust what they are doing in demo accounts because, in demo accounts, you will not feel the pressure or fear of losing money. But what if you also feel that way in live accounts? Then that could be a bad thing, so when trading in demo, those traders should treat it as live trading; they should feel the pressure and fear of losing the trade even if in a demo account. In that way, they will also become familiar with what it feels like to trade in actual. The movement in demo trading is the same as in actual trading, so in that way, you will be more familiar with how the market you want to trade in works.
People should, but people won't. They will be a lot more fearless when they are using demo, and some of them will get lucky by making some wild trades, and make a huge profit. Then, those same people will think that they can do the same at the real thing, and will try their chance, and instead they are going to end up with a lot of loss.
Demo trading should be acted like it's the real thing but we are not seeing people acting like that very frequently, that is the problem and I think it will keep being the problem as well. I think the reality is that we should consider the situation being very careful and very risky, we can't really make it work any other way. Demo can't be considered just like the real thing, it's that simple.