[Not US but] my local library service allows 300 simultaneous book loans, when I asked about why that number was chosen they said it’s so that people are not practically limited in borrowing. Given that typical loan period is 28 days even with 5 renewals you’re still looking at slightly more than 2 books read per day so that seems a reasonable allowance.

My first thought was car dependency. If it's a 20-minute detour or 45-minute dedicated round trip to stop by the library, you probably don't go that often. I read a lot as a kid, but I only went to the library maybe every other week. I distinctly remember the limit being 99 books (libraries are mostly run by counties here, so there are a lot of different sets of rules). I never ran into it, but I checked out dozens of books a few times.