Because it’s a monopoly created by international agreement. It’s like a phone number - it needs to be routable in the system, but if you follow the standards, you can get integrated into the system as a registrar
The top level domains are owned by countries - the UK has .UK, the US has .com and .gov, the UK has .io (because they stole it), but most countries have just one. They charge a fee to register a secondary domain, and the registrar can charge whatever they want to their customers to register on their behalf
This is just the centralized system though - you could build your own, AOL tried to do that through “keywords” back in the 90s
I thought the same thing. It’s a full answer - it’s not just “it’s the motherboard”, it’s “this is what is happening, we’ve reproduced it, and this is how you’d go about fixing it”