Now, it's true that the IPv4 internet also does this. But the IPv4 internet does this because they didn't expect to run into the limit (far fewer than 2^32 IPs were originally usable), and then they started working (halfheartedly, but whatever) on IPv6. Urbit has the limit because, as a philosophical position, they believe the limit is a good thing, because there should not be more than four billion people with full human rights.
https://urbit.org/blog/common-objections-to-urbit/
> A 32-bit planet is a tool, not a toy. Like a car, it's a device for a responsible and independent adult. There aren't 4 billion cars in the world, nor 4 billion independent adults.
> If you aren't an independent adult, and you don't need or even shouldn't have unconditional digital freedom (no one's 8-year-old daughter needs unconditional digital freedom), a moon from someone else's planet is fine. (Even most of today's independent adults don't complain enough about being Facebook's moons.)
This was written by Yarvin in 2016, when there were over 5 billion people over the age of 18 in the world, so at least one billion adults then were unworthy of first-class citizenship in Urbit's "digital city," and even more today - not to mention the 8-year-old daughters or 12-year-old daughters or 16-year-old daughters who need conditions on their digital freedom.