TF2's perceived popularity was always far bigger than the actual player count, especially when you compare it to Valve's other multiplayer heavy hitters like Dota 2 and Counter Strike, TF2 was always far, far behind in player count. TF2 was always Valve's testing grounds for Steam features, however it's now outlived that purpose and no one at Valve cares enough to work on it, so no significant updates leads to lack of player interest. Valve likely knowingly benefitted from the idle bots boosting the player count, had the shrinking player count been widely known about many people would have likely panic sold their inventories and sent the item economy into a negative spiral, which could have impacted the other item economy games, so they do everything to ignore the fact the game has been bleeding real players for a decade now and nothing short of a massive investment that they'd not see a return on will stop it, so basically lying about the amount of players is the easier option.

If you want to play a version of TF2 with an active playerbase, gets regular updates, and is about the gameplay and not spending thousands of dollars to play dress up with gaudy particle effects, then play TF2 Classic.