To make routing tractable.
MAC addresses are assigned by equipment manufacturers and henceforth remain static. The process of selling and distributing equipment essentially shuffles all the equipment around, so there's no pattern to them.
The IP devices in your vicinity, have an essentially mixture of MAC addresses.
Now when you send a packet out, it goes to your ISP's router. If routing was done by randomly-distributed MACs, each router in the world would need a comprehensive list of every MAC address, in order to know where to send the packet.
The routing tables would contain hundreds of millions of entries. And every time a dynamic node moved or get connected to network, you'd have to update every router in the world with the routing information.
So to not to get into such chaos, well structured IP addresses are used.