Unreal and Unity do much preprocessing and optimization of game content. All that takes place long before the game runs. Much of the smarts in Unreal Engine resides in Unreal Engine Editor, not the runtime. A metaverse client which displays dynamic user-created content from many creators doesn't have the benefit of that pre-optimization. There's not much instancing. Some content is badly optimized, has too much detail, oversized textures, or errors. You can't pre-bake lighting or shadows.

A metaverse client thus has most of the problems of an MMO client and some of the problems of a web client (browser). Games are normally pre-tuned so that the client does not face these problems. For a fully dynamic metaverse, content overload is the normal case, and frame rate has to be maintained independent of content changes. Think of what happens when you doom-scroll a web page. It's like that, but in 3D.

(There's a class of limited metaverse which works like a game level loader. Users build a game level, the level is loaded during a long preload period, and then you can use it. Sinespace, Breakroom, High Fidelity, VRchat, and to some extent Decentraland are in that category. You build your parcel offline, then upload, but you can change some object transforms in world and move things around. This allows some offline optimization. Clients for these systems are close to standard MMO clients. Dynamic stuff is tied to the frame cycle.)

Unreal could probably make their system into a metaverse-in-a-box product, with some functions that now run in the editor running on servers or as background tasks in the client. So far, they haven't done that, although Sweeney has talked about it.