Most of the legal streaming services require Widevine L1 certificates to display higher resolution content, and RPi boards are not "certified" hardware and thus can only see the content available to L3 devices (we fake a web browser). This is generally 720p HD although Netflix can often show 1080p content when available too.
Are you using adjust-refresh and a mode whitelist to change-to/use the native resolution/refresh of the media being played?