That's mostly because of the architecture of Unity3D being both efficient and productive and UE4 being much more so. They're radically different architectures than what three/babylon have. And the fact that they both compile through Emscripten is proof AAA engines can be built for the web.

We're not too far away from SIMD, Atomics and SharedArrayBuffer as well as OffscreenCanvas in the browser (they're all available behind experimental flags today). I can definitely see a newcomer building a web engine from the ground up and beating Unity/UE4 in performance AND productivity for not having their overhead. Its a huge undertaking but nothing impossible.

What I missed the most doing WebGL work however were asset pipelines. Open-source engines barely support DXT compression, normalized integers and whatnot. We ended up writing our own (very crude) CLI tool to compress DXT1/5, ETC1 and PVRTC as well as a KTX parser to load them at runtime. I'll see if I can make them open-source - they're still a bit tied to our custom in-house webgl renderer.