Because they're not on the dev team, which makes their ideas(even if well thought-out) ultimately pointless. Context matters too in this. Someone starts a thread inviting everyone to cook up new jobs, people pitch in and it's topical chatter of what-ifs. Someone starts a thread all by themselves for the sole purpose of shopping their idea around like it would ever actually be used? Tacky/pretentious as hell; for bonus points these threads will also contain superfluous tags in brackets trying to ape official threads. You usually don't get the same kind of reaction when people make suggestions for existing jobs, because they're mostly staying within the framework of the job and putting forth ideas to change abilities that don't quite work as one might hope.