Those people don't want that to happen again, they're afraid that they'll take another risk on something, that they'll risk believing in the game and then be let down when it doesn't live up to what they wanted. They feel it would be better to not try and not get hurt by disappointment than to take a risk and get hurt.
Then there's the fact that, when those people see someone really putting a lot of work into the game and being hopeful about it, they ridicule them or at least they try. They either tell them directly or imply that they're stupid for believing that anything good is going to come out of it and that they'll be disappointed when it all falls apart. This creates fear of ridicule and fear of ridicule keeps people from being idealistic about things, especially things that are seen as childish and silly and that is exactly how role playing games are seen by many people.
This is not the one and only cause for what you've described, video games have become a big replacement for roleplaying and that's effected how people treat the hobby but I think what I've said here plays a big part in this. Look at the young people who are new to role playing, look at how unfraid they are to believe in something and how immersed they get, they're not afraid to give the game a chance.