I’ve effectively worked on greenfield projects my entire time at Amazon:

1. New service within Prime to handle GDPR and other compliance related matters

2. Opensource CLI tooling

3. New AWS service

4. Another new AWS service

Maybe (1) doesn’t count since it had to operate within a preexisting microservice ecosystem, but in the rest we’ve had complete control of the product from languages to servers to cloud infrastructure.

I’d wager there are always new teams in most large companies that are doing greenfield projects, you just have to look for them and be willing to join.