[Toy Soldier]

By the time the dragon hit the ground, the Lannister army was basically fleeing / in disarray (as seen when Jaime was looking around the battlefield). The battle was already over, no normal soldier was going to suddenly decide to kamikaze Daenerys and Drogon. Except Jaime who felt he had nothing to lose and just saw red at his defeat.

But yes, none of the many thousands of Dothraki bothering to stop Bronn and the ballista was a bit of a plot hole. As soon as it was fired for the first time they'd have been all over him too. Easy fix too; they could just have made it that Dany spotted him from the air and started heading for him, and his first (and only) shot hit. It would be much more believable that he wasn't noticed setting it up... Rather than after actually firing the thing.