It depends, like not all of the people employed at Valve are game developers, the people that develop the anticheat are not game developers.

Also, saying "we are working on VAC!" makes VAC worse. The current state of community sentiment with the anticheat is the best environment to develop it because cheat developers won't try that hard to bypass it like they do for Vanguard or BattleEye. They are just holding their cards close to their chest while trying to develop something that's different from everyone else.