payload limits? can't be stopped by cheap defenses.

Flying drones do have counters against them - jamming (and while fiber optics solve that, they have their own drawbacks, like reduced range), being intercepted while in flight, and being stopped from detonating on important parts of the vehicle by additional armor. Plus, as insurgentrat said, there's payload limits, it's a lot easier to drive weight around than it is to keep it floating up in the sky, so in practice, the stereotypical really cheap FPV drones tend to have pretty pathetic payloads (I can't find the post now, but there was a case of a Russian tank surviving over a dozen impacts or something along those lines), and the fancier drones with greater ranges and payloads aren't actually as cheap as people assume.

There's only so much extra armor you can reasonably put on the bottom of a vehicle though (no way to fit an anti-drone cage on there), and you'll never be able to fully protect the tracks/wheels, since, by definition, those need to be exposed in order to actually contact the ground, so at the very least a vehicle could be easily disabled (which then leaves it vulnerable for other modes of attack - for example, during the 2023 Ukrainian counteroffensive the Russians used a lot of helicopter-launched ATGMs to actually finish off tanks and IFVs which had been immobilized in the minefields). Thus, until we invent Star Wars style hovertanks, mines will remain a major threat, and mines that can move around present a whole new dimension. From an earlier article on a similar topic: https://hexbear.net/post/6349607/6574868

“The only thing I feared as a tanker is the idea of a [Family of Scatterable Mines] minefield: something suddenly deployed in front of me from a family of deployable … munitions where there wasn't a minefield before,” he said. “The only thing I think scarier than that is the idea of a mobile minefield.” A group of ground robots “that has the capability to destroy me like a mine from underneath where I'm most vulnerable, but it moves to where I am. And if I don't go that way, it can relocate again and relocate again, deny terrain and deny me the flexibility of being able to be dismissive about a minefield, because that suite of solutions coming at me has the ability to follow me and compensate for tactical decisions I make — that’s the idea,” he said.