Even completely leaving aside the language/software diversity, the point is that the vast majority of web "frontends" are really just one thing (rendering the UI and responding to inputs) whereas web "backends" are frequently not really one thing (1 or more types of storage, 1 or more types of computation, responding to requests, perhaps event processing and scheduling, etc.)

I did not mean to imply that all, or even most backends are more complex than frontends -- only that the problem space of backends is larger and more varied (for typical web use these days, at least.)