Well, mostly because they are better for riding on the open road. Lighter, less bulky, bigger contact area, firmer grip. There are no real downsides in the context of road riding except shorter cleat life (and a pair of cleats every year just isn't a big expense).
If I want to walk around (eg. commuting), I do it on a bike built for it with flat pedals and just wear shoes made for walking. If I am fortunate enough to be able to travel by bike, I'll just pack walking shoes.
MTB SPD shoes seem neither here nor there. Ideal option for a MTB or gravel bike, sure.