Yeah, there's also a bit of a difference in that voting is connected to where you live, while selective service registration doesnt really matter where you live, just that you exist (and are male). They can find you if they need to.
Also, I've lived in multiple states with "automatic voter registration", and it's really a misnomer. All it is is an extra check box when you get a driver's license, so registration is: 1) not happening if you dont have a driver's license, 2) not happening if you choose not to check the box.
I've made this rant before, but every country has voter registration somehow. Even Australia where you are required to vote, it's still on the individual to enroll and update their enrollment whenever they move. Similarly, I've had Dutch people tell me they dont have to register to vote, and this is also not true because they have to register to exist at whatever address they live at, and voting registration is conferred along with that.
Some places let you prove you can vote when you go to the polling location, but that's still effectively the same thing.
The only way for it to be truly automatic is if you live in a microstate that doesnt have multiple voting districts, or the government surveils you to keep track of where you live.