For the vast majority of the community, this isn't an assumption. Skilled rules smiths and skilled designers are the exceptions in the community and not the norms. The vast majority of players, even pretty good players, aren't also skilled rules smiths or skilled designers. And ideally, they are capable of recognizing this and recognizing that content provided by the skilled designers is superior to what they could make themselves. I guess it would be a false assumption that all the best content comes from professionals and that there aren't amateur designers of equal worth, but one thing about professionalism that isn't a false assumption is that the marketplace tends to weed out the inferior work over time. Thus, the average community member is making a reasonable assumption that the professional stuff will be more reliable than picking something from the community. Remember, low skill designers can't reliably recognize bad designs. This is the reason that the amateur community often tends to be dominated by the lowest skilled designers who, suffering from a specific case of Dunning-Krueger effect, assume that they are actually very good designers. It's not that there wouldn't necessarily be a lot of good amateur content out there, but if you had a Wikipedia or something where content could be added, very quickly it would fill up with tons of ill thought out suggestions alongside the better quality stuff.
One reason you're wrong, why eg free BFRPG stuff is often more useful at-table than polished Pathfinder stuff from Paizo, is that professionals and amateurs work to different incentives. The Paizo stuff is made to fill a word count, look pretty, and maximise sales. The free Basic Fantasy stuff is written
much more focused on at-table use.