Per Microsoft's best-practice recommendations on the Power Platform regarding environment strategy, your tenant's default environment should be renamed and re-purposed for "Personal Productivity" (i.e. used for individual exploration and the productivity of every-day tasks) and is not intended for production application use.
This recommendation is for good reason. These default environments are “special” in several ways:
Default environments cannot be disabled or deleted.
All licensed users are added automatically to the “Environment Maker” role and cannot be removed from that role.
The Environment Maker role can create resources within an environment including apps, connections, custom connectors, gateways, and flows using Power Automate.
You cannot create or restore backups of the Default Environment
No backups? No way to control who your makers are? Concerns with implicit connection sharing? These are all good reasons why an organization should absolutely not be using their default environment for production applications. Instead, organizations should be using a series of DLP policies to limit the capabilities of the default environment, and ensure production apps are built in a production environment that can be governed properly.
However, currently extending a SharePoint List Form using a Power App only supports the default environment. This limitation is entirely counter to Microsoft's own environment best practices. The ability to select the environment where your Power App will reside is critical to being able to build secure and compliant SharePoint-centric solutions using the Power Platform.
Here are a few reference documents regarding best-practices on environment strategy: