I see caching is disabled by default (except for self-hosted runners).

What are the reasons for not enabling by default?

Nearly 30% of PyPI's downloads come from CIs (via BigQuery). Given 47.1 billion total downloads per month (via PyPI Stats), at least 13.8 billion downloads/month come from CIs.

Last month, PyPI used 105.5 PB bandwidth, which at list price would be around $12.5 million per month (via PyPA Discord). Fastly very generously gives PyPI a 100% discount on their bill.

At PyCon US this year, Fastly said over 10 years, they have served 4,682.1 billion requests and 2.41 exabytes of data:

If astral-sh/setup-uv enabled caching by default, this would significantly help ease the load on PyPI and Fastly, plus it can speed up runs by several minutes.