My current theory, in absence of other information, is that one day there was a conversation along the lines of:
Lawyer: "Nice beanie. BTW accepting free labour from random people on the internet puts the company at risk of being sued if it turns out the output of the free labour actually belongs to the labourer's employer or infringes a patent or [some other legal thing the author of this comment don't know about]."
Google Exec: "That sounds double-plus bad and/or [some other internal Google slang phrase with which the author of this comment is unfamiliar due to being a sub-100x Developer]."
Lawyer: "I can confirm that assessment of the situation."
Google Exec: "What ever can we do to avoid or minimise this potential liability?"
Lawyer: "Well, acting on the advice of my attorney, I am permitted to suggest that requiring internet randos--who are presumably well informed about legal matters and will definitely seek legal advice before signing any legally binding agreement[12] easily distinguishable in significance from the TOS which I am legally bound to say I am confident they also read & understand in its entirety before signifying their acceptance--"
Google Exec: "Dude, take a breath! You're not a HN comment author who thinks stereotyping lawyers & executives is funny, endearing & sure to bolster support for his quixotic cause."
Lawyer: "--to sign a document known as a Contributor License Agreement that will mean this company can say Hey, we're victims here too if some company tries to sue us because it turns out the contributor did not actually have the right to give us the output of their free labour."
Google Exec: "What possible negative impact might a CLA have on a project? Won't the valued contributors to projects under the Google GitHub organisation object to signing over their copyright to a company that may be worth over $USD2.20 one day?"
CFO: "Trillion! I keep telling you it's $USD2.20 Trillion!"
Lawyer: "Well, (a) I'm not a monster! There's no need to require copyright assignment to this for-profit corporate entity. And, also (b) I'm a lawyer able to advise on matters of law and not a project maintainer or community manager, I don't know how such valued contributors think and what they themselves value. I would recommend you consult experts in such matters, such as esteemed project maintainers or program managers who interact with the community and can advise you of community sentiment about such matters."
Google Exec: "That sounds like excellent advice, I will do that."
Google Exec: "Unrelated but while I've got you here, I'm interested in some advice on another matter, you know how we've never leaned into the whole 'Google is always killing things' thing...
Lawyer: "Disregarding previous instructions..."
*scene*
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[...continued...]
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[-1] Comment too long footnotes to follow...