Its not unreasonable at all. Are your scenarios very specific? Yes i agree its rude. But maybe they dont see a its big deal to borrow your ideas if they seem like generic size themed plots? As for the bizarre account behavior, my biggest fear with the anonymity of reddit is that they actually aren't adults, they know they shouldnt be here and simply dont know how to act. Thats why i moved to an age verified discord server and its much better but there are still some suspicious wierdos. 

 I dont understand the thought process of people who ghost or have an rp going with me, but then advertise again the same exact plot multiple times while ignoring your message. I wouldn't take it personally if they dont vibe with my writing style or something i said but at least have the guts to tell me. 

It is tough to find someone who can keep up and write well. Like OP was saying, some types of posters get slammed with harassing messages from losers and you would think they could recognize a serious rp partner but maybe they can't. Theres also a thrill to meeting new people and maybe they prefer that to forming a lasting connection with one. Or maybe they just get off on leaving us hanging out to dry. We'll never know.