Cost to extract and re-create what planet earth creates from biology from which benefits and is essential for humanity isn't achievable in space. You can get rare metals and materials but the insects and organic materials essential for life aren't out there in any reasonable way .. unless you know of a planet that has those ecosystem services.
Plus you've got to deal with the biological offensive you're going to be exposing yourself to. Imagine a whole planet of microorganisms against which we have no immunity or medical experience. Do we even have an antibiotic that would be effective against the new lifeplans we'd encounter? The Spanish colonizers of the Americas died in droves, and likewise the indigenous population; all this from bacteria in lineages which our bodies were familiar with! Now imagine what you're going to experience on a fictional planet with an ecosystem that could support our bodies. We'd be living in hazmat suits for generations, or would have to burn the planet to ash and start fresh.
Are you sure humanity is going to get there, and survive, before they run out of resources to build what's needed for such a thing here on Earth?
Also even if we do that, there's only a tiny fraction of the people on Earth that will be able to go on that journey. Most of the people born here are definitely stuck here.
There are predictions that some of these things could start happening as early as 2050 (specifically the extinction of edible seafood one), and most of what I mentioned maybe happening by 2100. Is the majority of people here still alive at that time going to be off this planet then?
Any resource limitation problem we have on Earth will be unfathomably harder on an exoplanet, short of hitting the moonshot jackpot of an exact Earth-equivalent with matching biome. Not agreeing with OP on the direness of resource exhaustion today, but planning to resolve ex. climate change by moving off-planet is like planning to pay for your heart surgery by winning a lotto scratcher.
