Because the playoff crucible is the ultimate test of a player's ability. You want to focus on the regular season because Curry is generally worse in the playoffs.
Why would you adjust for shot attempt discrepancies? Lebron's ability to be a higher volume scorer is a testament to his superior ability as a scorer. The funny thing is the difference in volume is a single shot attempt-- Lebron averages 20.7 FGA in the playoffs for his career while Curry averages 19.5. Those early high volume Cleveland playoff runs hurt Lebron's efficiency (playing in an era where spacing wasn't a priority) and yet he's still as efficient in the playoffs for his career as Curry is (despite Curry playing fewer games in addition to playing in a more favorable era).
These are all excuses and attempts to decontextualize areas where Curry is clearly superior. I will duly note you think the regular season is of little importance going forward.