Except we made an exception for citizens that reside in Washington DC. They have no representative in the senate, but were given 3 electoral college votes for president and vice president.
So we totally can (and have) extended the right to vote to citizens living outside one of the 50 states to vote, we just won’t for Puerto Rico. :(
You make it sound like Puerto Rico is some tax haven where they don’t pay the federal government anything, but Puerto Rico pays more in total federal taxes than 6 US states.