5E's 27 point buy is customised version of default array, which harkens back to 3E where WotC did a lot of math to determine most common results from the popular 'roll 4d6, drop the lowest' method. Which in turn was the default system used in 2E, ie. the basis of BG1&2.
Put another way; roll 4d6 drop lowest = default array = 27 point buy.
This all said, 3E recognised lower and higher point buys for different campaign styles. So yes, if you reroll like crazy, you need to offer say 35 point buy to be comparable.
However 5E streamlined and simplified wherever it could. BG3 went even further and simply dropped the rolling method, I'm guessing because of multiplayer feedback, some players can't handle disparity.