I don't understand. Why not?

I can't recall ever having a problem. The rain fly extends out over the doors, so there's very little, if any water hitting the doors. It would take quite a bit of wind for the rain to blow straight onto the doors.

If I'm concerned about wind/rain, I'll set the tent up so that the end of the tent is facing into the weather. The rain fly extends almost all the way to the ground on each end.

(edit) I've camped out in a handful of pretty nasty storms, and I think the worst thing that happened was that I got a little mist from water that blew under the rain fly and came through the mesh. It was probably a little irritating at the time, but certainly not a big deal.

Jamie

For the same reason I take off my shoes when I get inside the tent. Inside the tent is a clean(er) space, I want to keep it free of mud and dirt as much as possible. My pants and jacket, and definitely my boots, are often going to be dirty at least on the outside during a multi-day camping-focused ride. Plus, if I'm setting up camp right after or during rain, my gear is going to be wet as well - I don't want wet gear inside my dry tent.

As a secondary concern, the point of having a two-person tent for one person is to have space to move around inside. Filling up the inside of the tent with my gear, luggage etc. during a camping trip is super inconvenient. I might as well have a bivvy sack.