TF lets you deal a lot of damage when you get it
It's not that you're doing less damage lategame, the difference is that Tanks usually have good defenses at this point so you can't kill them and ADCs are dealing WAAAAAY more damage than you lategame so you can't really compete with that.
Because of this, fighters will usualy go Triforce > Tank (Hecarim, Skarner) or Triforce > Bruiser (Irelia, Vi). Triforce is usually all the damage these champs need to kill squishies which is why they need the durability to not blow up in one hit.
TL;DR Fighters that build TF > Tank/Bruiser fall off lategame because they do less damage to tanks and don't have enough damage to compete with real carries
NOTE: I didn't include Yasuo, Jax, Xin Zhao, etc. Those champions generally build multiple damage items after TF so they don't fall off
Yasuo building TF is kinda aids though, I think he is way more balanced when his optimal build is full crit + squishy. This warrants a Yasuo nerf and not a TF nerf.
Same with Ekko, he should be nerfed before TF gets nerfed. He's like a traditional Triforce champion except way more slippery and he has no drawbacks which is stupid.