The better question, IMHO, is why do they not reach it?
Some fall to the TPK and start a new game.
Some are campaigns where the DM only wants to run for a finite period, and ends the game when the DM is no longer willing to continue.
Others end when the players elect to go in another direction, either ending the group or having the group play a new campaign with a new DM.
In my experience, the games ends most often due to DMs ending it (rather than PC deaths or players giving up). DMs burn out, they desire to play more than to DM, or they get too busy to run a game. Players are often very invested in their characters by the time they get above 10th level and - if the DM has crafted a game where they have stories to tell - the players really want to finish their stories. If the DM is just running dungeon after dungeon with no sories behind thecharacters beyond episodic challenges, they still are eager to get to that next ability that comes at the next level, or to find that next powerful magic item...
Games ending before the highest levels are a missed opportunity. When I'm a player, I seek to avoid it my trying to engage with the DMs world and support them in making it fun for them as a DM as much as it is fun for us as players.