In the end it takes the protagonists almost KILLING THEMSELVES from the sheer strain induced on their puny bodies trying to keep up with Jiren for just a FEW MINUTES to do ANYTHING to the guy.
All that just to weaken him down to a level where they have one shot at pushing him out of bounds. Even this last-ditch effort of last-ditch efforts is entirely predicated on, one, knocking out Goku, Vegeta, and Freeza just to move Jiren a few feet off the stage, and two, hiding 17 in the rubble and hoping like hell that Jiren doesn't target him. Because if Jiren finds him, or if he's able to avoid their last shot, it's over. They'd have NO chance in an actual fight without tournament rules.

This is verily equivalent to the Cell arc on the first fight of Goku and Cell, where the ring out rules were still in effect. IMAGINE if the arc ended when Goku fell off the stage, only for SS1 Gohan to come in when Cell was exhausted and push him off bounds. Then Cell just said I lost, and the credits started rolling. Would that really show Gohan or Cell's full strength and potential? The same applies to Jiren.

Then came the Broly movie, and as I mentioned before, it was completely overshadowed by the previous feats in the ToP.
They literally had to write UI off the series because it was simply too powerful for the subsequent threats. Broly fell short of being a threat. He got ragdolled by a Zamasu tier fusion. And his petty little fans just couldn't cope with it and expeditiously left the threads forever. Nowadays the remnants of his sympathizers are constituted of occasional pathetic tourist, and a single FOAZ.