But that's you playing, right? Not someone attacking you?
My team is composed of three Luneras, Ayaka and Basch. Sometimes I guarantee a kill on first turn, sometimes I don't.
When I don't, I set up the AoE cover with Basch so the enemy team doesn't have a chance to wipe me (there are dozens of teams with 3x Ashes/2x Dark Veritas etc that can easily wipe you).
Having a tank is insurance. It's a big deal for people that care about getting the best rewards in the arena. Global weekly arena rewards are trash so I don't give a fuck about it, JP weekly arena rewards are godlike so I want to guarantee I'll never lose my streak.
If you are finding a lot of teams with tanks here it's because people care about not losing their streaks when they don't get the kill on turn one. Simple as that.