Can't claim child as dependent

This is more of a general tax question

My son was 18/19 years old in 2021.  Lived at home full time.  Went to college part time.  Earned approximately $8000 in 2021.

 

According to the rules I can't claim him as a dependent because he didn't go to school full time and earned more than $4300 in 2021.  Even though I paid for his housing, medical, medical insurance, auto insurance, etc. - basically "supported" him all year.

 

It seems strange that $4300 earnings and not going to school full time all year are the two deciding factors.

 

Just curious if there is more to it.