Then it wouldn't be the motherboard: the board it slotted in to would be the motherboard.
Not only that, but the high-speed connections, e.g. between the processor and RAM, need to be as short as possible, otherwise they wouldn't be capable of high speeds at reasonable costs.
If processors on cards were that good an idea, Slot 1 would still be in production.
Motherboards don't fail so often that it would be a cost-effective design.
Do you want more reasons...?