Yes, I also fail to understand... why we need 40+ MP cameras. Sigh...
You don't need it, but it sure is nice when you have it. The only way to get it right now is MF. There are trade offs for MF - and they are huge:
Lens options - let's be honest, there are 10x more in the 35mm world than in the medium format world
Lens length - 200-400mm zoom, 800mm lenses - 35mm has this covered, 645 shooters just got a 2x extender
Shots per second - up to 14fps - need I say more? Phase does a dark frame equal to your exposure, and Hasselblad does 1fps
Speedlites - TTL is your friend, and doesn't exist in in the MF world, so your wireless remotes are just 'pop'
So, for people who want these high MP images in the 35mm format, the ability to use these advantages is huge.
But, 40/50/60/80mp images are AMAZING to work with, and when printed are a whole different ball of wax. There is a market for it, but there is an associated cost to it. I can do a 24x36 print, and my resolution is 300 pixels per inch - off of a native file. Yes, you can interpolate up to this, and even higher, but side by side, a MF image at size will look better.