Blame religion.
It is perfectly possible to build a generation ship to reach for the stars and colonize planets thousands of years of travel away.
Just not in the way that we think of generation ships...
Cryo-sleep is how science fiction solves this problem, but the only known state of human that can survive cryogenic freezing is the human embryo.
a crew of around 6 females, and 40,000 frozen embryos, a new generation of females is implanted via ivf and then born through live birth every 25-30 years.
Long story short, spreading humanity out across the stars with our current technology would require massive shifts in how humanity sees morality.
Building a colony on mars, populated with people who have free will, and not risking the wrath of the religious right, sure... that's easier.