Because it's not a very worthy fight. Since Kanno asked us to ignore in-game mechanics (even then, the numbers for boss bosses is eclipsed by everything in the other games anyways), Trema/Paragon don't have many powers featured in the cutscenes. There's speculation that they could take Spira by storm, but so could Sin and Vegnagun. Though they have great capabilities, every single one is hampered by their personalities.

Vegnagun is too scared to do anything without Shuyin (and Shuyin is not at all stronger than Tidus); Trema and Paragon are in a similar position, only for fiends. Although it would appear that I'm arguing a moot point; FFX-2 was clearly ignored simply because it won't hold a candle to any of the other games, both in or out of battle.

I think that individually, the only games with comparable Final/Extra bosses would be FFVIII, IX, and X. The problem with the other games is that their "extras" are unions of powers. Old allegiances die hard, but they also die because they can't be maintained.