Because Google attracted a lot of employees who care about corporate ethics with its "don't be evil" reputation. That reputation has faded as people learned about cases when Google didn't live up to it [1] but there are still a _lot_ of Googlers who want to hold Google to the old standard.

If there were another large tech company with a reputation for great ethics, many of these people would probably go there instead. But in spite of a lot of disillusionment, Google still seems to be the best of the FAANGs IMHO.

Full disclosure: I just left Google. I'm taking a break before looking for something new, but if someone knows of a tech company that is as ethical as I once thought Google was, I'd love to hear it...

[1] The wage-fixing cartel, several exec sexual harassment complaints and Andy Rubin's $90 million severance, double-speak around Project Maven, the complaints behind the walkout and ultimately Google firing the walkout organizers, etc.