Mordenkainen's Tome: what monsters are in it?

[Unoriginal]

I see no reason why anyone should read anything this book has to say about elves or dwarves. I haven't seen anything that seems remotely interesting in that respect

That is your opinion. Personally, I love what they told us about those, and have found it very interesting.

and I think it's silly to pretend that their histories should be in any way related across different settings.

This, however, is just plainly ignoring 5e lore. As they have repeatedly said, all the official D&D settings happen either in the same Material Plane or in a plane connected to it, even if each world is in its own Crystal Sphere which obeys to slightly different rueles. So, yes, all their histories are related, because mostly eternal divine entities sharing a relatively unified workspace.