Backend code lives together better too. It's trivial for me to set up a frontend web server that points one URL to a handler in Python, another in Go, another in Java, and whatever else I like. Said frontend can also do a lot of useful abstraction over things like login and authentication, and even to a limited extent authorization. Backend does not compose perfectly but it composes together reasonably well. This architecture may be problematic for other reasons, but it isn't intrinsically a huge problem right out of the gate.

Frontend, you largely have to pick a team. Trying to run multiple frontends in the same page has a number of problems, if nothing other than each of them gets so large that even just one can impact performance, but they will also fight over events, and you can't cross the streams, etc. The whole structure is more winner-take-all, by its nature.