If it's not devs, then explain how a) some UE5 games are more optimized than others and b) why were devs still making poorly optimized ports before UE5?
Like, are we going to blame Cyberpunk's crappy launch on UE5 now? Batman Arkham Knight? Monster Hunter Wilds? Dragon's Dogma 2?

Some games actually run fine on UE5, like The Finals.