Even in the case of services like Netflix and Amazon Prime Video, the DRM measures are a significant impediment to casual infringement, and in particular to "ignorant" casual infringement by the kind of person who would quite happily put a whole show on their YouTube channel and then write something like "No copyright intended" in the description as if that meant they hadn't just flagrantly broken the law. For any big name, mass market title, there will probably be a way to acquire it illegally relatively quickly after release for those who know where to look, but there is a very long tail of infringement beyond that point and that's what they're trying to control with these kinds of measures.
As a pertinent illustration, a while back Google started adding a download icon to the default toolbar for HTML5 videos in Chrome. It didn't do anything you couldn't do before just by using the context menu and saving. And yet Internet forums were swamped by complaints from people who made DRM-free videos available on their sites in the immediate aftermath, because viewers assumed the presence of the icon meant it was now OK to download the videos to keep or share arbitrarily. My businesses typically don't employ any fancy DRM schemes for content we provide to customers once they've paid and logged in to whatever system they're using. But that week, after spending an insane amount of time chasing down copies of multimedia content that was for paying customers only yet suddenly started popping up on every hosting/sharing system you can think of, we did implement a really dumb "breaking change" in how we served videos, just to get rid of that icon, and it solved our problem overnight.
As a pertinent illustration, a while back Google started adding a download icon to the default toolbar for HTML5 videos in Chrome. It didn't do anything you couldn't do before just by using the context menu and saving. And yet Internet forums were swamped by complaints from people who made DRM-free videos available on their sites in the immediate aftermath, because viewers assumed the presence of the icon meant it was now OK to download the videos to keep or share arbitrarily. My businesses typically don't employ any fancy DRM schemes for content we provide to customers once they've paid and logged in to whatever system they're using. But that week, after spending an insane amount of time chasing down copies of multimedia content that was for paying customers only yet suddenly started popping up on every hosting/sharing system you can think of, we did implement a really dumb "breaking change" in how we served videos, just to get rid of that icon, and it solved our problem overnight.