Is veneer white oak really that much more expensive than solid paint grade maple

I imagine your "solid paint grade maple" is also veneer just like the white oak. If you have face frame cabinets, the face frames will be solid, and the doors and drawer heads likely have some solid material.

So yes, using veneer covered plywood and MDF is normal.

Yes, much more expensive. Using veneers is standard.

Veneers are far more stable. Which is why they are industry standard for wide pieces and panels. And shelves are never done as solid wood. They may have a solid nosing on long shelves but most of the time it’s veneer with edge banding.

The upcharge also depends on what cut the veneer is; quarter-sawn and rift cut are more than rotary or straight cut. Our cabinets are quarter-sawn. The end panels and shelves are a veneered plywood. The doors and drawers are solid.

Rift cut white oak is the hot cabinet material presently. Half of that also has a more expensive glaze finish also. $$$.