Hi Lane,
I can think of lots of reasons:

1. The mortgage on the property is paid off but the owners needed to move for work, etc. Doesn't take "more money than sense" to keep an empty property if there are no mortgage payments to be made.

2. The owners moved to a smaller property in a particular school catchments area, but intend moving back to the original one once their kid leaves school.

3. It was an inheritance and the beneficiaries don't desperately need the money and haven't yet decided whether or not to move back into what may have been the original family home, and don't want the hassle of renting it out.

4. Retired owners who have bought another property abroad but want to keep a UK bolt hole in case of needing or wanting to come back here.

5. Holding onto it as an investment for retirement.

6. Someone in the armed forces who is buying in readiness for when they leave, but don't want the hassle of renting it out.

7. Parents buying a property near a chosen university for their teenage kid(s) while they can afford it.

and many other reasons that I can't think of right now. "More money than sense" isn't one them.
Thanks
Leonardo