IP addresses sharing a route have a common prefix. This is not true of MAC addresses. They are allocated essentially randomly. If you wanted to route solely using MAC addresses, every router in the world would need a lookup table containing every MAC address, route aggregation would be impossible

That's not /the/ reason why a MAC address is involved. It's because that's the address for a physical device at a lower layer in the stack. As others mention, IP is media-independent, it cannot depend on a lower tier addressing scheme without becoming fused to that medium