A lot of people ride mountain bikes, a fair amount have pretty expensive bikes, (not talking just people that show up on forums and take surveys, most don’t) most use flats, and most use what ever cheap brand of athletic shoe they wear everyday. Even the most popular brand of mountain bike shoe, is a very small market share. Some big companies just don’t care about small market shares.
I have two mountain bikes, my new bike an Evil I put together myself, my old bike a Giant with a $1000 worth of mods on. I only ride flats, and I wear cheap Sketchers (I pay about $30 a pair for) and find they work as well for me as expensive 5-10s or any other dedicated mountain bike flats shoe. If I was riding clipless it would be a different story.