Why take the ACT if already took the SAT

There was a recentering of SAT score in 1995, so that a 1980s SAT score is likely equivalent to a higher score after the recentering.
https://files.eric.ed.gov/fulltext/ED563025.pdf

The 2016 SAT redesign also changed score equivalencies:
https://collegereadiness.collegeboard.org/pdf/higher-ed-brief-sat-concordance.pdf

Probably a combination of “the test is easier” (to get a high score on due to the recentering, whether or not the questions are actually easier) and increased preparation and multiple tries.