I’ll play “Devil’s Advocate” for a minute here …
There are some very legitimate reasons why a player could leave an in-progress M+ keystones, things like power outage, ISP crapping out, IRL emergency (familly, fire in the house, and so much more), and we cannot know what happened without waiting and actually talking/writing about it with the concerned person.
There are also players, called “griefers” or other such derogatory terms that actually take pleasure in preventing other players from enjoying the game.
And finally, there are players out there that care only about their M+ Rating and Win/Loss ratio, the size of their epeen on Damage Meters, and their own ego. Those types of players will leave a key at the first sign of things not going in the perfect way, and my “ignore list” is getting at least three more names everyday because of that behaviour.
But even if I find that way of playing toxic, counterproductive and detrimental to my enjoyment of the game, it’s STILL a perfectly valid and legal way to play according to that long chart of acceptable behaviours that almost nobody ever reads.
Short version : too many valid reasons for a player to leave a M+ run in the middle of things, not enough manpower to track all the people that leave and ask for their reasons, and there’s already penalties in place (cannot join another group for 30 minutes if you just left mid-progression). And it would do nothing against said toxic players that enjoy the suffering of other people.