Many of them are, actually. See:
https://www.spur.org/A fair number of tech companies donate to SPUR, which does policy analysis/writing, advocacy, etc., including Dropbox, Salesforce, Netflix, and others.
That said, I was disappointed to see last year that Google apparently joined about the month I did (as opposed to, say, a decade ago) and were donating at some small level (sub-50k/year, Microsoft was in a higher donation bracket ffs). The amount of fighting Google is willing to do in Mountain View while ignoring the rest of the bay astounds me.
https://www.spur.org/publications/urbanist-article/2017-08-2...
https://www.spur.org/join-renew-give/business-membership/spu...