Peter B wrote:
In a nutshell: weight!

Should have guessed!

what problem would making the rim wider solve?

I guess I assumed that wider rims would be stronger (assuming comparable
quality of build/materials) which might be an issue for mountain bikes.
And like the OP I also assumed that a tyre could be too wide for a rim
and be at risk of coming off.

I don't object to having my illusions on this subject shattered.

d.