Maybe a full engine or framework is not needed, but I think games are similar enough that there's quite some logic to be reused. Maybe some libraries at least.

Stuff like path-finding or collision detection is fun to write, but chances are someone else did it better.

While the APIs are simple, it is what engines can deliver on top which is important. For example, one fairly important thing, imho, is shaders and structured passes. While there are lots of good and easy WebAPIs, structuring a graphics pipeline and glueing everything together is quite hard, especially for a beginner. So an engine delivers the abstraction. Just tell it to create a cube with light and marble-like material surface, and a good engine will deliver. Not something any WebAPI can, or should.

Also, it's not true that there is no performance in the browser. Throw in WASM, and you can easily create a 3D FPS. Maybe not on the scale of Battlefield or CoD, however playable and in the browser while looking nice and modern with cool effects.