There’s a leak somewhere. Once the bulb is installed correctly it becomes a seald unit. A crack or seam glue space will allow the exchange of air getting moisture in.
That moisture isn’t a big problem unless it get on the bulb which will explode.
For one customer of mine who was in a big hurry to get home, I used box tape over the missing front of the lens to keep out the water splash and they made it home without having to repeatedly replace the bulb .
Yours may not be so obvious. Just a stone nick or the seam glue saparating will let in moisture.
Find the hole, seal it, dry it out, and reinstall.
But to find a leak you can’t see will require removing the lamp ( and the bulb ). Fill it with air pressure or water and look for the leak.