Why are the front of the floorpans so susceptible to rust?

I'm in the process of tearing my car down to the body to do a complete rebuild, and the only rust on the car I found was in both of the footwells on the driver and passenger side. It's the area where the floorpan meets the firewall. The rust is pretty bad. I can almost pass my whole hand straight through both of the holes between the pan and firewall.

Why so much rust just in these spots? Is is simply due to water collecting in that area from wet shoes in the rain and snow? It certainly sucks though, that's for sure.

Any tips for repairing this? I can weld, but I wasn't sure on the gauge of metal to use. The pans aren't too thick, but the firewall seems thicker. 18ga?

Yep. It's been in the northeast throughout its whole life. I did notice that seam you mentioned. It's right under the firewall and there was probably 5lbs of road debris (and rust) caught in the small area. I can see how water could just sit in there and eat away at everything. Thank you Ford.