MAC identifies which device. IP identifies where that device is located.
I know your name, but not where you live, so I can't send you that 100 dollar bill I promised to send to you using the postal service.
You are correct about that within only 1 LAN you know exactly where each device is. So technically, only a Mac addres table would be enough to get your data wrapped in a frame across the LAN.
But there is a catch. All networking devices and PCs are build using the TCP/IP networking model. 1 model to rule them all. That model requires you to wrap an IP address inside a frame. So without an IP address you can't send frames. Without frames you cant use the MAC addresses to get your data across.
Technically we could build a second networking model, solely for within LANs that ommit the IP address requirement. But that takes ages. Then you need to convince the IT industry to make hardware and software that takes your model as their own. Its just too much hassle.
Your question is like asking why do we need cars to travel over the highway? Why not walk, its close by. Well its against the rules to walk on the highway. And you end up in jail.
Technically you could push for a change of law. But you need to write that law. Taking into consideration every other law that is connected to it. Also the entire highway grid is designed with the idea in mind that no pedestrian makes use of it. You got to convince the public, get elected, get enought votes, get your bill pass congress and the senate. Its just too much of a hassle.