Is there a way when configuring alerts for a subsite's pages in that you can select everyone who has permissions to the subsite? This way if someone joins or leaves the subsite the owner wouldn't have to manually add or remove them from the page alerts. Thanks so much.
Not easily: since it's a subsite, there is no OOB distribution list like there would be for site members of a root team site. If the users are in a SharePoint group, it is possible to use flow to get the email addresses of the users that are in the group.
Unfortunately, there is no OOB action to do this, and it relies on calling the SharePoint REST API. A blog write-up with the specific instructions is here. Note, the specific action used is "send an http request to sharepoint". This is important, because the older "http" action has become a premium action.
Alerts are designed to work with users, not groups or distribution lists.
Assigning alerts to a group just isn't part of the design.
You can use flow as a workaround.
In addition, it's a by-design behavior. If you need more information, you can go to user voice and post about it.