If you're looking for a reason the housing market is so tight, perhaps the Baby Boomers are to blame.

For generations, after the kids grew up and moved out, many parents put the big family house up for sale. According to one market watcher, that doesn't appear to be happening nearly as often nowadays.

"The traditional 'empty nest' storyline where parents would be downsizing is not happening," Svenja Gudell, Zillow's chief economist told CNBC's "On the Money" in an interview. Research from the real estate data company found the inventory of homes for sale is the lowest in four years.

Gudell said a contributing factor to fewer homes on the market is more baby boomers are staying put in their "home sweet home," rather than downsizing.

"Baby Boomers have to live somewhere, "Gudell said, "but they are having a hard time downsizing and finding that available inventory. And so it's tough for people to move into their homes and that is causing that disruption in the normal 'move-up' market."

As a result, Boomers aren't able to find a home that's smaller and cheaper than the one they're in "that would entice them to move," Gudell explained, "so for many it does not make sense to sell."