That's common. If the truck lives anywhere salt is frequently used, the inner door lip/seem needs to be frequently sprayed out during the winter months and coated with something like fluid film / white lithium grease.

The previous owner probably never did either. My truck is an 08 and has zero rust on the inside of the doors and the roads here are salted from November - April.

The truck pictured isn't bad now, but it'll just keep getting worse unless it's corrected.