I just replaced the crank on my gravel bike yesterday. It doesn't get ridden much in the rain and it hadn't been ridden in months because the gravel has been under snow. It was definitely swampy in there. So water gets in and doesn't leave. No drain hole in the bb shell. I have never seen a plugged drain hole, it doesn't have to be very big, 5-6mm. Fortunately, it's an all city and they have an anti-rust coating inside. I'm convinced it would have been dry if there was a drain hole. I also took the seat post out to make sure it wasn't corroding, and it was fine. Still had grease for quite some distance down the seat post. Who knows how the water gets in.
I should probably cut the seat post shorter.