Earth has a lot of people living on it, and any effort at repair is just as likely to wreck it. You can't "colonize and terraform Siberia" without turning the US into a scorching desert.

Mars has 0 people living on it, and is already uninhabitable. We literally cannot make it worse for anybody, and are in no danger of wrecking anything we need to survive by trying.

I would much larger we try planet-scale geo-engineering on Mars, then try planet scale geoengineering on Earth (and we're already flirting with that because of climate change).

There is a wealth of information to be gained from attempting dramatic changes to a planet's condition, and establishing a biosphere, which would be extremely important for any attempt at large scale Earth biosphere repair.

We could if we had the political will. There are terraforming-like approaches that would e.g. reduce global temperature, but good luck getting the political consensus to actually put them into practice.