This is a much-discussed topic and you can easily find tons of information on it. For one thing, not trying to expand into new places goes against our nature - not just as humans but as living things. Every species we know of collectively tries to find and adapt to new habitats. A long-term practical reason is that inhabiting more than one planet ensures that our species will survive planetary disasters, like the asteroid collisions we know have repeatedly caused massive extinctions on Earth. Or the volcanic upheavals 250 million years ago that wiped out most life on Earth. We know these things happen again and again, and if this is humanity’s only home when the next one hits, game over.