Why can't humans migrate to other planets?

Humans can immigrate to other planets, mainly because of technology and funds, so they can't immigrate or explore other planets at present. However, I believe that in the future, as long as the technical problems such as nuclear-powered space engines and the recycling of substances necessary for daily life in space are solved, and the problem of money is solved, it will be almost no problem to realize interstellar migration or explore other planets. (but limited to: water, gold, earth, Mars. It's too far to reach)