So why has per-capita student spending doubled since 1990 (adjusted for inflation) without any increase in test scores? Why haven’t we been spending the money wisely?

Student to teacher ratios have continuously decreased and are about half of what they were in 1960. Data on the results is mixed: https://www.brookings.edu/articles/class-size-what-research-...

Because we have also increased the spending in "un-education" (entertainment, social media, college sport...) ?

What's your own theory ?