The government gets its power from the people. That still doesn’t mean I want my neighbor to have a fully operational 155mm howitzer.
Why not? Do you think they'd shoot it at you? Do you also not want them to have a nitro jet boat, a 60' RV, or some other kind of recreational equipment?
Just because they're legally permitted to own a 60' RV doesn't mean they must be allowed to park it on the street.
In most states, your neighbor is already permitted to own a grenade launcher and 50-caliber machine gun under certain regulatory restrictions, so do you object to that particular adherence to the Constitution as well?