Space colonization is simply jumping third or fourth steps removed from the dream of making our Earth more livable and more ecologically diverse, or learning how to build and make an acrology.
From that perspective, it's no wonder why people think it's a waste of space, because they saw no meaningful purpose. Meanwhile, we're doing the opposite of terraforming Earth, by making it more unlivable everyday.
If space colonization leads to mining resources from asteroids and offloading the environmental costs of such mining from earth or get us building materials strong enough to give us a space elevator or space factories sending their waste into the sun, you wouldn’t be saying this perhaps.
From that perspective, it's no wonder why people think it's a waste of space, because they saw no meaningful purpose. Meanwhile, we're doing the opposite of terraforming Earth, by making it more unlivable everyday.