We can say the same about you.
A depleted key doesnt remove your shot at timing that instance at that key level through any other dude with the key.
Yet you want to remove key depletion because it will stop people leaving?
Are you even reading the replies?
There are differences in both types in content, and thats it.
You also dont instantly end up at your 4th mythic boss without progressing through 3 others.
The same way you dont loot a m+15 key as soon as you hit lvl 60 with your character.
Getting to 10/10 mythic requires progress through earlier bosses.
Nobody starts on Sire on mythic and skips the rest.
M+ keys: designed to “progress” through key levels and “get better” while doing so.
So you start at m+2 and depending on your skill or gear you progress through a few keys until you can time m+15 or find a group for it.
M+ is about progressing, and key depletion means you didnt make it, which would be a “wipe” in mythic raiding, although you get to retry your m+ key at a lower level because Blizzard understands you dont have to “start at an m+2 key again”.
Raidingwise, you get an instant shot again at said boss you wiped on. Yet, the boss doesnt become easier, you just have to play better.
And if not, come back next week.
Although for m+ keys: come back next week doesnt exist, so a m+15 key which is truly easy atm due to affixes wont be that easy next week. And will take up to x many weeks before its the same affixes.
There is your raiding / m+ explanation and why it is different.