Apparently it's a thing in Subarus Of A Certain Age, because there are a number of internet forums discussing wetness inside the headlight assembly. Baby's only happens on the driver's side, but it has killed two headlight bulbs so far. The permanent fix of replacing the headlight assembly is pricey, and Baby is 8 years old - a mere child in Subaru Years, but still....
This issue started last year, after a HOT, HOT, HOT summer, and coming home from work to park Baby in the garage. Let's see: Hot car in extremely high humidity outside all day at the office, driven a short distance and then parked in a much cooler garage. Condensation City. Yes, obviously moisture is getting into the headlight (hello, it's old) but it's not sloshing with rainwater, it's just dripping condensation inside the lens.
I think Baby is a cactus, not an orchid. She has lived 8 years in harsh outdoor conditions and these transitions to the cooler but still humid garage space don't agree with her.
I took Baby to the local shop for bulb replacement (Son-in-Law has enough on his plate) on my way home from work today, and asked them to give me an estimate on replacing the headlight assembly. I love this shop - the guy said, "It's not worth it. Just keep buying a new bulb every six months." He priced it out, showed me the cost with both Subaru and aftermarket and their very reasonable labor charge, and said it was ridiculous. I could drive the car for another six years with a new bulb every six months before I spent what the headlight replacement would cost, even with their very cheap charge for labor. I said that's what I was thinking too, thanks for confirming.
I'm going to experiment with leaving Baby in the driveway again and see if the headlight bakes dry. Worst case, it doesn't work and I buy another bulb in six months. No biggie.