Because it would require new electronics which is tightly integrated into a very large and complex board.Why, if Tesla is selling CCS1 adaptors in the US do they not do upgrades on older model (May 2020) Tesla Model 3s? And... does anyone have any more specific info on when Tesla will do the retrofit than just 'early 2023'? Or where in north America that it is doing so now?
In general, auto makers don't provide upgrades for new functionality between model years. Tesla has done it sparingly in the past (most notably actually developing a new battery pack for the original Roadster as a thank you for early Tesla customers), but they are under no obligation to continuing doing it in the future. Ask Apple about upgrades...