Why do so many take more classes than needed in HS?

It depends.
In Texas, college admissions are decided by class rank June Junior year. Senior year course selection is taken into account depending on the major but for a non selective major, a high rank is sufficient. If they’re top 10% (or top 6%) they’re okay.

However, it isnt sufficient if students are aiming for UT, a selective major (especially CS, Engineering, Business), Honors college, or an Out of State College.

Overall, in order to be competitive at selective 4-year colleges, you need

  • 4 years of English, with 1 AP if possible
  • Math through Precalculus Honors or Calculus (honors, AB, BC; Precalculus + AP Stats is okay for Humanities majors but Calculus is prefered, and exected for STEM/Business.)
  • Foreign Language through level 3, 4 or AP
  • Bio, Chem, Physics + 1 AP science (one of these for STEM, APES if Humanities/Social Science)
  • 4 years of History/Social science, including 1 AP if possible
  • an art class
  • other classes that reflect your academic strengths and interests

That being said, highly selective colleges expect 6-8 APs TOTAL. Taking 6 APs senior year is meaningless wrt college admissions and is likely to cause burn out. So, a senior year with all 5-6 core classes (as presented above) including 3 APs, plus a fun class you’ve always wanted to take (like Culinary Arts, Current Events, Theater Tech, Shop…) is 100% fine.