This is because Google Play Movies is a marketplace where one can rent or buy movies and TV series, while other streaming services follow a subscription model.
Google, however, is looking to change things around as it is seemingly planning to offer hundreds of free ad-supported movies through Play Movies.
A teardown of v4.18.37 of the Google Play Movies APK for Android has references to Google adding hundreds of free ad-supported movies to the platform.
It is unclear if the company intends to make its entire Play Movies library available via an ad-supported model or if it will be limited to selected movies from a handful of studios.
At the moment it is not known when Google will roll out free ad-supported streaming of movies on Play Movies, though an official announcement could just be a few weeks away.