First, though Steam has sales. PlayStation Store also has sales. Second, console games have historically been more likely than PC games to support same-screen multiplayer with two to four gamepads, and if you have more than one gamer in the house, one copy of a $60 game that supports multiple gamepads is cheaper than three copies of a $30 game that requires a separate copy per player. Third, if everybody were to wait for the sale instead of buying in release month at full price, publishers would have no money to continue to fund development of high-production-value games.
Not to mention the fact that, unlike PC games, you can often find dirt-cheap used physical copies of console games on Amazon and other sites. A while back I bought a used copy of Battlefront for PS4 for $10 on Amazon. Good luck getting it for sale on PC for that price.
And even if you could, big Steam sales only come a few times a year and many devlopers don't even support Steam (including big names like EA and Ubisoft). By contrast, cheap used copies of console games can be found anytime.