Unfortunately for every gallon of ridiculously expensive petrol you buy you get a small amount of water mixed in with it, which then lays in the tank and rots it from the inside out, so even though as you say the tank is well protected on the outside, you can't escape the rust and debris that builds up on the inside.
However all cars suffer with the same problem so unless the tank is particularly bad proper filtration will keep it away from the EFI, and regardless of even a new tank, eventually the same thing will happen again. It can be a lot worse in tanks that have been left standing. Hence part of the reason for the move to plastic tanks.