I see this behavior surprisingly often as well. Any explanation I offered would just be conjecture, I'm afraid. I have some guesses about why people do this, but they're just guesses. I think it would make for an interesting psychology study.
Anyway, I'm mainly posting here to offset the toxic comments I see in response to your question. I, too, have been there. I've been attacked by onlookers for making suggestions, with the naysayers backing off only when the project leads decided that my suggestions were good ones. I've watched other people get attacked similarly, sometimes when I was a newcomer, and sometimes when I was the developer. It doesn't seem to matter if you're making suggestions for someone else to implement or offering to do the work yourself; some people seem just as likely to sling mud at you either way.
The internet has no shortage of obstructionist personalities, and the communities that gather around software projects are no exception. It makes me sad every time I see it, because to me, it is the antithesis of open software development. When it happens, everybody loses.
The only advice I can think of right now is to accept constructive criticism of your ideas, but also don't assume that your ideas suck just because some internet troll says so. A lot of them are wrong.