They aren't tanky enough to stay in melee safely.
It costs them 2 attacks to disengage, (almost) half of their offence. 1 ki and the bonus action, which is the price of a flurry of blows.
So a feat that doubles your damage output seems pretty good.
A rogue loses half of her attacks to disengage, but they can get 80%+ of their damage from 1 hit, and are tankier thanks to reaction-halving a hit every round.
Maybe it comes down to styles of play that I'm unfamiliar with. It's got to be something.
What you're saying doesn't make sense to me.
Disengage as a bonus action is usually a bad choice for a Monk.
But...just walking out of melee doesn't actually help them. It is super easy for creatures in 5e to move around the battlefield. They will just walk up to the Monk and keep attacking. And if not the Monk then another vulnerable PC.
The Monk is better off Dodging here if they want to be defensive.
Mobile requires the Monk to attack each creature they want to walk past too. The Monk really wants to be hitting the important enemies with Stunning Strike. They can't do that if they're using up their attacks on the weaker creatures. Attacks which are not as helpful as a magic user's AoE attacks against those same creatures.
And if the Monk stuns a creature that creature can't OA anyway.
Movement gain is diminishing returns too. The Monk already gets a lot of movement so the added distance for Mobile is not as impactful as it would be for another character.
Moving away from a creature without taking an OA after you attack them is just not worth a feat. A feat is a big deal. +2 to Dex or Wis for a Monk is a huge deal.
At most Mobile is worth half a feat for a Monk. It mostly just duplicates what they can already do, just gives them more ways to do it.
I will never understand it I guess. If I had to take a feat for a Monk I would rather take Tough.