why do pro teams not pick jax into darius?

Jax needs to buy items to be damaging. Darius doesn't. You can just camp Jax or lane swap into Jax and he's useless. Darius can last hit with Q edge and build full tank.

cause fiora is open 90% of the time and oes the same as jax, only better.

But mostly, jax works way worse in laneswap scenarios and isn't as strong in 2v2s as darius and overall has some problems dealing with ganks. In an isolated 1v1 lane, jax will straight beat Darius, but he's far more item reliant, so if he falls behind, he's prettymuch done for, unlike darius.

And in proplay, camping such a lane or laneswapping is rather common, it's not rare to see games in which both toplaners have anout 15 cd at 10 min, and in such a scenario, jax is 100% usless. Which is also a reason for why irelia is seen so rarly, she also crumbles if her trinity gets delayed to long.

Because what does Jax offer to the team?

Because just is hard to play,only useful after 35 minutes,and your team can't 4v5 too long