How would a networking stack with no hardware addresses even work? The next hop needs a way to reach back to you, before you can negotiate anything fancy like passwords or certificates. Even IPv6 SLAAC starts with a hardware address.
It’s also worth mentioning SLAAC does not strictly require a hardware address to function. For example, Apple devices implement newer standards from IETF to generate random but stable addresses that don’t reveal information about the hardware addresses (https://support.apple.com/guide/security/ipv6-security-seccb...)
Not if you need to send a message to $thatUniquePhone.
Over simplifying considerably, but if a land line places a call to a mobile, the "220-1234 calling for 220-7890" message enters the network. The `220-7890` phone number needs to map to the unique modem address so you can look up which tower the call setup data should be sent to. If - by sheer coincidence - I also have your MAC address and am attached to a tower 3 states away... which tower(s) do you forward the call setup data to?!
Whichever one has most recently communicated with the user in question (based on the credentials or certificate provided, in the original example)
If you have a wired connection, this makes the MAC completely superfluous. The concept is sort of still valid for wireless connections (or of course for "wired" connections where you have multiple devices physically connected by the same wire, a bus, where the concept originated). It should be rethought.
Note that this is only about last-mile/first-hop. Once you scale past a single LAN segment, routing is mostly layer 3 until you get to core Internet infrastructure which uses ASNs and BGP. At the very least, it is probably sufficient to say that routing across a WAN is all about IP, but if you zoom in on parts of that network of networks, the underlying technologies often use other routing mechanisms internally that don't get exposed to the other parts of the WAN.