Why finishing open world RPGs leave me feeling empty, and thoughts on how to fix it

tl;dr: Whenever I finish an open world RPG, I'm left with a sense of emptiness and futility, that all the equipment I've collected, and the character I've built up, is now useless. There needs to be some sort of sustainable challenge at the end for you to practice it all with, maybe by turning the end game into a rogue-like?

So all through an open world game, I'm running around collecting things, often to build up the perfect armor set, collect the perfect weapon loadout, or get vehicles or build up my character's move set.

If you're a completionist, then by the time you reach the final boss, it's often kind of easy. You end up overpowered for it, and it doesn't even feel like a challenge.

But more so, whether the game just ends, lets you keep playing after the ending, or "rewinds" to just before the final sequence, once you finish all the side quests, there's just nothing left to do with all the stuff you just got.

One of the most egregious examples of this is the >!motorcycle!< in the Breath of the Wild DLC that you don't get until there's literally nowhere for you to go. You can explore the world and just fight random enemies, but it's usually no longer any challenge at that point.

It leaves me with a feeling that I collected all this stuff and can't even use it.

Occasionally, there is some sort of arena where you can fight very strong monsters or bosses, as in the Hollow Knight DLC or Horizon: Forbidden West. That can help, as it at least provides a challenge, but it tends to be a bit too different from the core gameplay?

One idea is to expand that monster/boss arena mechanic so that there's a portion of the game that becomes a bit of a rogue-like, where there's some degree of exploration still, along with the enemy-fighting?

What are your thoughts? Or is this just fine, and games should just end?