There aren't so much servers with 16 slots, but right it's often around 12, and 12 persons on a not that large DM map is chaos.
This may be because people follow tutorials for setting servers, and don't take the time to set some cleaver value for it. DM usually requires less slots than CTF I think.
Also, it's a max limit, but it's not always reached, so this setting doesn't matter that much.
Finally, spectators take a slot, I'd prefer to have the ability to distinguish players slot and specs slots, because I agree if people join a server and spec, or wait, or talk ... while being unable to join.
Admins are just too lazy, that may include myself