Please explain why an environment is required about deploy-pages HOT 1 CLOSED

TL;DR: security.

Long answer:

Traditionally Pages has been published from a source branch in a repository. Users have been able to restrict who can publish a Pages sites (even with write access on a repository) using branch protection rules.

When building and publishing a Pages site from a workflow, we explicitly removed this "source" branch limitation. It does not make sense to force the workflow to run from a specific branch when there are so many viable events which could trigger a workflow. We not only require an environment to be specified when doing a deployment, we require it to be protected. This requirement is simply our security mechanism for allowing users to have a finer control over Pages deployments like they always had with source branches. In addition, specifying an environment means we automatically create deployments on your behalf. A lot of investment are being made on that front currently.

from deploy-pages.