Looking for a better understanding between an IP address and a MAC address

This isn’t really a homework question but if I’m asking it on the wrong subreddit feel free to tell me which one would be the right one to go ask this thanks.

I’m currently going through the Introduction to Networks class on Cisco’s netacad and have difficulty on grasping the use of MAC addresses if every device already has a unique IP address. I can understand that if the frame stays on the LAN that it has been sent from it might be easier for the switches to find the corect destination device but in the end if it’s the IP packet that knows where it’s supposed to end up and every end device has it’s own IP why even bother with the extra step of adding the MAC?

I also understand that if the end devices destination is not on the same LAN as the source device, the destination MAC address becomes the one of the router that ships the packet to the other network and not the one of the end device itself but with that comes my confusion of why is there not just a single fixed IP address for each network and then a unique MAC address for each receiving device instead of having a unique IP and MAC address for each device?

It gets a bit more confusing to me when I think about the fact that there is only about 4 billion unique IPv4 addresses and there is about 7 billion people on the planet (even though probably more than half of them don’t even have access to electricity let alone the internet) plus all the servers and other devices that exists, if IPv6 has been in rnd since the 90’s how has it not become standardized by now?

I hope this made sense to some of you, it has been bugging me for the past couple of weeks and since it’s an online course I don’t really have a teacher that I can go ask these questions to. The teachers from my actual classes might be able to help but the few times I did asked things somewhat out of their fields they seemed almost as lost as I was so since reddit has been helpful in the past I figured I might give it a shot.

If you made it all the way through thanks for trying to help it means a lot :)