I think it's actually the opposite. American universities receive less research funding from external private sources than universities in the European countries I'm familiar with. The difference is probably due to the culture of charitable donations. Europe has a tradition of private foundations funding arts and sciences, while Americans make donations to universities.
In 2021, academic R&D spending in the US was ~$90 billion (https://ncses.nsf.gov/pubs/nsb202326/funding-sources-of-acad...). Out of that, 55% came from the federal government, 25% from the institutions themselves, 6% from nonprofits, 6% from businesses, 5% from state and local governments, and 3% from other sources. The share of businesses looks normal, while the share of nonprofits seems low.
In 2021, academic R&D spending in the US was ~$90 billion (https://ncses.nsf.gov/pubs/nsb202326/funding-sources-of-acad...). Out of that, 55% came from the federal government, 25% from the institutions themselves, 6% from nonprofits, 6% from businesses, 5% from state and local governments, and 3% from other sources. The share of businesses looks normal, while the share of nonprofits seems low.