Developers should not touch test or stage environments under the non-production category. Introducing new software has risks of introducing functional or security problems.
As test should be like production, so too should dev be like test. Isolating infrastructure encourages that as new services are being added, they are figured out in dev first. And it helps IT administrators when the compute and network infrastructure follows the same design in each environment. Networks are cheap to set up in the VPC cloud concept.
Bend these rules to suit your organization. A developer may put on their analyst hat to troubleshoot an issue in test. However, they should respect any change control processes, and limit significant changes to the agreed on update procedures.
For those wishing to follow along, the referenced diagram comes from the GCP console's cloud-setup wizard. As the overview mentions, "This guide is intended for large organizations." Which makes sense for a design that sets up a handful of administrator groups, and assumes several projects.