That's not true.

Everyone gets a 30-year mortgage because that's the pricing model. Everyone sells or refinances because nobody lives in a house for 30 years. When they sell, they take their equity and use it for a downpayment on another 30-year mortgage. You keep doing this until the kids are whelped and you're ready to retire and then you sell the place and use your equity to live off of in a small downsized place in Tucson or Ft. Lauderdale or some shit.

Or at least, that's what you're supposed to do.

Now, you buy a place and have your brother or a roommate move in. And then you hope you can sell it when you need to move to another city for work. Then you need better schools so you move somewhere else but you can't carry two mortgages so you're renting your place at a loss and hoping you can find somewhere soon but now your parents are sick so they have to sell their place and move in with you and now you're all renting and it's pretty fucked up.

but.

In the ideal world that the lenders and real estate agents want you to believe in, no you would not pay off a mortgage in 15 years. You'd start a new one. To them, you'd ideally have started three.