ok seriously why does everyone keep making that assumption
there are other reasons to have popular servers
for instance having a constantly topped-off server means that you definitely have enough people for the server to function properly so you can stop making guesses about how x amount of players will react to y server feature and just observe what happens when x players react to y because x players are already here on your server, and maintaining a certain level of fullness means that your server will keep itself topped off and you don't have to maintain xty players
also you can gauge gamemode popularity, get more criticism faster, bugtest faster and (in cases of deathmatches) shoot more dudes because all of those things are functions directly related to the amount of people currently playing your server