The labor cost is often huge. If you don't prevent fraud at the $500 level you'll have two problems: Lots of repeated fraud at the $500 level, and people trying to get away with fraud at the $5000, $50000, all the way up to FTX levels of fraud. So you need multiple people in different depts to agree to fund something while other people watch over to make sure the people being funded are not the spouses or shell companies of the approvers. Meanwhile there's paperwork costs to make sure the donation paperwork gets to accounting so accounting gets their tax deduction.
At a "really big company" the labor cost of paying an invoice might be $2000. Now does the average programmer get more than $2K of annual value out of urllib3, given the alternatives (which could involve up to architecting around needing it?)
Before you laugh and say "no way" to the $2K figure, imagine spending half of an hour long team meeting of ten software devs getting paid $100/hr but overhead etc costs the company $150/hr for them to attend that meeting, that's $750 of labor costs just to have the dev team possibly agree to support urllib3. Then its got to go up the chain to someone with signing approval, someone has to chase the paperwork to make sure it happens, frankly $2K is on the very low side of the cost of getting an invoice paid.
Commercial non-free software is "put up with" where its not that some rando piece of required software costs $50 thus the cost of the company including labor and overhead is $2050 and they're happy to pay it, its more similar to if you don't dot all the Is and cross all the Ts that software will cost $10M in some crazy lawsuit, and $2050 is cheaper than $10M, so they pay up or they'll get a huge lawsuit or more likely someone will get fired after the audit, and I'd rather pay $2050 for something worth $50 if it prevents me from getting fired and better yet its not my $2050. But nobody never got fired for not paying for urllib3, so nobody's paying urllib3.
Companies will pay for support and I could see a large company paying $10K/yr for a written guarantee if a dev at that company runs into a use problem or bug with urllib3 then an actual dev from the project will help them up to twenty hours per year or similar, even if no one asks for help. In contrast feel good donations are very expensive so they don't happen often, so trying to run a business off feel good donations doesn't work too well..