"basically everyone who's serious about OSS requires a CLA."
Nonsense. Red Hat don't require a CLA for any of their projects. The Linux kernel doesn't have a CLA. Libreoffice doesn't have a CLA. Mozilla don't require a CLA. The Python CLA might have that name but is actually just an explicit statement that the contribution is licensed under certain terms, which makes it entirely unlike the Apache or Canonical ones.
"I'm not sure what the history is there or why their lawyers came to a different conclusion than everyone else."
Most projects with CLAs have them because they see other projects with CLAs and it seems like the safest option. The common justification of "We need to know that the contribution is authorised" is more elegantly solved by the Developer Certificate of Origin approach, and the secondary justification of "What if an original contributor revokes permission to distribute" is only a problem in countries with string moral rights and a CLA isn't going to save you there anyway.
The FSF has copyright assignment for strategic purposes (they want to be able to launch enforcement lawsuits without requiring that the original authors all register their copyrights), and Canonical have a CLA because they want to be able to release proprietary versions of their code without granting anybody else the same freedom. The majority of cases are just entirely unnecessary.