Curbless shower question: why do I need a mortar bed?

I understand the need to slope your shower pan, but why can't I do this with plywood? If I'm installing a linear shower drain the floor can be two pitched planes, easily achieved with plywood and some kerdi EDIT: thank you u/MortyBingle for your explanation.

To paraphrase his reason: You can use plywood, but it would be stupid. It likely won't save you much money and is a lot more work.

I'll be installing a mortar bed.