Third reason could just be developer being lazy fuck which doesn't want to implement any kind of monitoring for how much people are online. Also if your game on Steam, Steam tracks it anyway (although it tracks only Steam users).
The online platform should have this info available on the store page of the game.
Quote from: FatherlyNick on September 30, 2019, 07:51:18 AMThe online platform should have this info available on the store page of the game.Steam has currently playing count in community hub of game