You’re correct that it has nothing to do with militia anymore.

It’s because the Supreme Court ruled that it has nothing to do with militia and everything to do with letting everybody have whatever guns they want.

That’s a relatively modern viewpoint. It’s the result of huge amounts of money being spent to campaign in favor of letting anybody have all the guns they want.

As a result, the United States has more gun violence and more gun murders than pretty much any other developed country. It’s not even close.