If you own a building that could sell for $10m to the right buyer after a few months on the market or $2m if you had to fire sell it tomorrow, then that building is worth $10m, not $2m.
Assuming the interest rate is around 4-5% as it is now, that means the building is worth a couple hundred thousand less than $10MM. That’s the definition of “slightly less” in the exact scenario you set up.
If you think a couple hundred grand doesn’t matter you are free to wire it to me at your leisure.
Now make six months into six years or sixteen years and you understand the problem better.