The osi model has 7 layers of connection to form a proper internet connection.
The MAC address exists but doesn’t leave the physical network. The MAC address is used to physically connect your computer to the router, and it defines your piece of hardware.
The IP address can change, because your computer can connect to different networks.
If you tried to route everything with a MAC address, (which isn’t possible, but for arguments sake we will pretend it is) the problem is that when you take your phone with its MAC address off your wifi and on to your work wifi, Where would the registry be? How would the Internet know how to find your phone? Do you just log into one giant global registry so that everyone can find your phone when they are trying to communicate with it? That would be a giant fucking database and everyone would always be trying to use it.
Routing is a big and complex problem, and these things didn’t work with ipv4
They do work better with IPv6. IPv6 adresses don’t need to change like ipv4 for a bunch of reasons.
From a philosophical level, the Internet was designed for people to be anonymous and make relatively anonymous connections. You wanted to be flexible enough that you can just be assigned a new number and work with that new number quickly.
This is a really simple explanation, and I got some basic facts wrong just for ease of understanding, but the principals are correct.