A lot of modern games have put a lot of time into their gameplay loop, and in part, this is why a lot of modern games feel like work. Focusing on this too much really can crowd out spontaneous fun. A gameplay loop also does not guarantee that a game is fun. Your loop might be: deploy --> shoot bad guys --> loot things --> come home --> process loot. None of this guarantees that the game is actually fun. Maybe the enemy design sucks, or the weapons feel bad, or the game just feels grindy.

In this way, it feels a lot like modern movies: in a lot of cases, cinematography seems to be some sort of objective science which has mostly just improved. And nowadays even a fairly bad movie will have great cinematography. It's just that the writing / plot / acting / etc. are quite poor.

That is, a proven gameplay loop can still fall flat quite badly. Easy examples would be all the modern hero shooters / looter shooters.

It's also worth noting that the definition of what constitutes a "gameplay loop" is pretty loosely defined. 1993 Doom clearly has a gameplay loop in the strict sense of the word: start level --> get weapons / ammo --> get keys --> kill monsters --> exit level. But this feels much less mechanical and gameified than your average modern game which almost certainly incorporate things such as RPG mechanics / stats / level-ups / FOMO events, etc. The latter feels much more artificial and forced, whereas Doom feels like "just playing a game."