Probably to cover their a$$. If it blows up on you their lawyers will point out that you exceeded the use by date. No reason why it should though. It's not much different to any other compressor tank. Unless it got really rusty inside or it was repeatedly over-pressurized.
I recently spruced my portable up with some paint. Was getting some surface rust from weather exposure. Having seen pictures of aftermath of shop airtank explosions, I'm fairly wary of rusty ones. (the rust that gets you is inside where you cannot see it). Have never undersood why all air tanks are not galvanized inside. Small cost increase in the overall scheme.
I still have my little Bostitch pancake compressor circa 1990. I empty the tank when done and am sure to drain the water. Don't use it often now with battery powered nail guns.