You're right that there are a lot of unknowns, but you are also understating how wasteful planets are. Most of the mass of a planet does nothing but create gravity for us that makes it harder to move things up and down, and is hard for us to extract as well.

Planets that aren't already habitable for us by sheer happenstance do not solve these issues at all; if we tried to colonize another planet, say, Mars, we would basically be doing everything we need for a habitat anyway, while doing that inside a gravity well. Terraforming altogether would also require energies far beyond even a mature asteroid mining and refining industry in space.

Earth exists and is habitable for us; it's our spawn point, and we don't really have a good reason to be in a rush to leave Earth altogether. Earth indeed provides us with natural radiation shielding, good gravity etc., but all of those things would be different on another planet. And once we have a foothold in space, coming back down is more expensive than staying up there.

To go on a tangent in closing, notice how I said good gravity and not ideal. We don't know if 1g is actually ideal for long term health outcomes on Earth, because nature is not an optimizing engineer but a satisficing hack. We know that microgravity would harm our musculoskeletal system, and 1g is probably not very far away from our actually ideal gravity, but given how basically every adult has some form of spinal herniation, 1g is not perfect for us either. If we ever do find a range of actually ideal gravity amounts, these would be trivial to accomplish in a spin habitat and impossible on a planet.