It’s been mandated federally in the US on and off since 1917 and the current continuous requirement has been in place since 1980.
If it’s mandated, what’s the perceived issue with it happening automatically?
The Congress-critters snuck automatic Selective Service (Draft) registration into the recent NDAA.
[Conscription is slavery](https://mises.org/mises-daily/battle-over-conscription).
Discussion
Because a person can decide not to register (and risk getting caught), or can appeal it, or can instead register as a conscientious objector. This bypasses all that, or at least would make "un-registering" much more of a hassle.