Because the US government has requirements about what it accepts.
You can't just implement whatever algorithm you like, then sell a router with that to the government. It must comply with whatever standard the government decided to adopt. And given that the government buys a lot of things, it wouldn't make economical sense to make equipment you could never sell to them.
This snowballs, and effectively sets a global standard for encryption. Sure, in your home you can do whatever you like, but the important thing is the security of the internet as a whole, and all of that is made of hardware and software that wants to be able to be used by the US government, and as such must support whatever standard it decides to adopt.