Mars is 0.3 G, which is just as hostile for long-term habitation. No human child conceived, born, and raised in that environment would have the ability to stand on Earth without an Aliens-style rigid exosuit. Their heart might not even be capable of pumping blood to their head.

Nobody knows what 0.3 g does to the body because we've spent 20 years on the ISS doing zero-g "science" even after the most important question was answered:

Q: What does zero-g do to the human body?

A: It's really really unhealthy.

Hollow out the asteroid and spin it.

This wouldn’t work, asteroids are like gravel there isn’t really anything to hollow out and even if you did when you spun it the asteroid would just shoot these rocks out in all directions.

That tech is decades in the future.

Is it? Assuming an asteroid of appropriate structural strength, it's just a question of time + propulsion. And acceleration adds up pretty quickly when you're in a vacuum and have all the time you want.