Because putting a beacon on a stealth aircraft is like installing a flashlight onto an invisible car.
Even if people can’t see the rest of the invisible car, they’ll see the flashlight and track that instead. At a minimum, such a feature would be ‘default off’, and never default on.
A stealth aircrafts literal design is to be invisible to enemy radio waves. The last thing you wanna do is you know… Emit a radio beacon.