Do you mean the comfort level is always 18 no matter what you add or remove, or that 18 is the highest comfort you seem to be able to get?

If the latter then the short answer is: because 18 is currently the maximum comfort level you can get without building around a Maypole (which not every map has, you generally have to either wait for its seasonal addition as a buildable item or use devcommands to temporarily add it to your hammer options).

Before you try pointing out reports of it being higher, take notice now that such information is waaaaay out of date - from back when different rug types could stack for comfort (which they no longer do).