The whole point of an externality is that the people doing the damage aren't the ones paying most of the cost. There's no rational limit to the cost an economic actor will impose on others for the sake of some small personal profit.

So we can't just say that the cost of damage must not be so high, or we would already be cleaning it up. There is no "we," there's individual actors and messy politics, full of entities making personal profits who are good at influencing politicians.

On top of that, the costs aren't even paid by us today, they're paid partly by our future selves decades hence, even more by future generations, and impossible to exactly predict. That makes it even easier to focus on today's personal profits and ignore the externalities.

Put another way, this is a public goods problem, where individual efforts make little impact unless everyone does them. Everyone is better off if the problem is fixed, but individually profits more by freeloading. The game theory says individual contributions go to zero. Once again, the only solution we have is political.

And finally, the technology to clean up the mess for $100/ton is just now beginning to emerge. We're not doing it already because we can't do it yet.

Most climate economists estimate the cost of carbon at several hundred dollars per ton. So yes, the cost is higher, and yes, we should scrub the air, once we're able.