For a lot of PhDs that I knew, the problem was that they were already spending 100-200% of their time on their main work, which made it impossible to spend any

For a lot of PhDs that I knew, the problem was that they were already spending 100-200% of their time on their main work, which made it impossible to spend any 20% time working on an entrance into what they really wanted to do (typically research, not uncommonly tools that would have made theirs and others' work easier, faster, etc.). I (and various other former Googlers) can attest to it being very difficult to do what you want to do within an organization. But that's the way it is in any company. Google just gives you access to a lot of other great people, resources, and perks that would otherwise be effectively unavailable.