Cuz they re simply unsustainable, brutal, harsh, etc. Like Soviets back in 70s tried to land spacecrafts (a series of spacecrafts known as Venera similar to Apollo) into Venus, only for the whole equipment to be melted into pieces due to it being corroded by the non-ending 24/7 sulfuric acid rains. Landing in Neptune=Instant freeze (It's Antarctica, but 1200 times worse). Space is not a star war fantasy toy lol. It's basically a unforgiving hellhole filled with lifeless unforgiving planets, asteroids, bright burning nebulae, stars and.ever stretching wasteland of void called "space". Colonizing space and other planets is like setting up a city in middle of Siberia.