Because submodules is hard to understand. They require that you understand git and that you can add one level of abstraktion.

Once you done that, they är great!

Also hard to deal with. Lots of operations leave submodules in unclean / out-of-date states.

In general / light use, yeah, they're great. Unfortunately, they have a very large number of edge cases where they essentially require either a) everyone to be experts in the edge cases, or b) tons of new tooling (because existing tools won't take these steps for you).

That is... until you try to remove a submodule.

I like sobmodules, but the whole feature needs a lot more polish before it's widely adopted.