Why are Jedi so weak

You just need to star your Jedi - also your team comp is not optimized. You need to get Jinn to 7* and Yoda as well for that boost in health that makes them arena viable. Ideally you'd have Anakin in there as well, not Barriss, so that you have some sort of theme. Either be fast and high damage or be slower and tank / heal. That current setup doesn't have enough power / speed to kill stuff, but at those star levels and with those characters you can't tank very well and take damage either. Essentially your growing squad at this point has not primary strength - which is expected as it's in development. You'd want QGJ as your leader - because right now you have a slower / lower health Jedi team with Barriss as your lead, but still trying to use Yoda / Jinn. Look to build a team that has a clear strength and make QGJ your leader. That will make Lumi fast and Yoda really fast - and of course QGJ will then be able to attack / lead with that assist. If you had Anakin now you have 3 toons at light speed. Old Ben is also a great Jedi. Aayla and Kit are solid too, but not top top tier. I realize they are chrome only, but they are Jedi and good ones if they ever come available.

I've definitely seen all Jedi teams take #1 in arena and they are deadly: QGJ leader, Lumi, Anakin, Yoda, and Old Ben is a very capable team comp.

I think if everyone had all the Jedi - if the best ones were easily available - this type of question wouldn't really come up. Jedi in this game as a group are very good and have the best depth and options.