Because when the constitution was written there were 4 million "Americans" there are now 331 million.

When the constitution was written, it took 30 seconds to reload a single shot musket that had a maximum effective range of 100 yards. An AR15 can dump a 30 round magazine as fast as you can pull the trigger, be reloaded in 3 seconds, can hit targets out as far as someone can realistically see.

The problem with the rights is that they no longer function as intended. When they were written 20 civilian men with muskets was not significantly less dangerous than 20 soldiers with muskets, the idea that the people could stand up to a tyranical government was sound. Its not anymore. I dont care how well organised your anti-govt militia is the ATF or FBI could fuck you up without much trouble if you wanted to try them.

I say this as someone who thinks going shooting is enormously fun, I love the engineering, science and talent involved in it as a pursuit. But those "rights" mean mass shootings of innocent people dont even make the mainstream news half the time.