Are they supposed to be the middle man between you and God?
Yes
Didn’t Jesus die specifically for that?
Uhh, not exactly. I think catholic god sealed heaven off after Adam and eve did the thing. Jesus came down and died to fix everything and open heaven back up. I assume the waiting room was getting full. He also died for all sins ever, but you are still born with original sin and have to go to confession etc.
Asking saints to intercede is just asking for personal bullshit. Different saints were known for different things, so being experts on those things they would be the best to hand your prayer based on that thing to God.