why do people have cactus plants at home? what's the reason for that?

They like them.

They're easy to grow as houseplants, and they like plants.

Because they want a low-maintenance plant that's hard to kill. Or because somebody gave it to them.

They like them, they think they're interesting.

It's an alternative method of security. Some people choose knifes of baseball bats, but I gotta tell you a cactus spike stings like something else.