So many reasons, all soundly based in a mix of statistical and anecdotal evidence, already advanced that I have no new insights to offer from my advanced age nor as an "outsider" in terms of devotion to mountain biking as a sport!
And that is the point - there ARE many reasons. Some specific to this site and its members, more much wider.
On sales here I think there has been a move away from complete bikes - focus on parts only really is for true devotees (and many of them already have more "stuff" than they could ever need!
I no longer have the energy to seek and build - for bikes FB Marketplace is my go-to source. Some amazing bargains, but for 5-10 years old bikes.