Tommy D. ,
For me, it's three reasons:
My wife's family, my wife's family, and my wife's family.
I do hope to downsize to a townhome/condo once we are empty nesters, since our priority is travel. Since we plan on having the house paid off before we are empty nesters, I'm hoping to be able to buy a nice condo and sock an extra 200k in the bank, to boot! Plus, if everything else falls apart, we always have that ace in the hole of moving some place cheaper.
(no attempt to take a logical stance for the pay off mortgage vs. invest debate - we just like the emotional security of no house payment)