Why do some families rent and not buy?

[Jonathan Feliciano]

For a while now, I've been hearing from multiple real-estate investors that "families make the best tenants"; they're more likely to keep the rental property in good condition and they tend to rent for a longer period of time, usually several years.

But what I don't understand is why do some families rent---and not buy---a property for themselves? I feel like it would make more financial sense to buy a property you plan to live on for several years. So, why rent?

I'm curious to hear what you guys have to say about this.