Presumably the best thing for Netflix is to have a happy userbase, so why do you assume it wouldn't optimize for that?
People want joy, education, entertainment, etc. from watching a video.
But there may be other ways of appealing to people (addiction, insecurity, base stimulation) which boost engagement but which do not give users what they want.
Obviously on even slightly longer time scales, users will gravitate toward services that do not trade their health for engagement, but equally obvious is that many of today's apps are not optimizing for long time scales.
Netflix cannot make you happy
If I don’t care enough to finish a movie I may as well start a new one. At the very least it’s not a clear choice.
Why not let users choose? Because, sadly, it's about money and not about users.
They are doing it to increase “engagement” and so more people will stay on their site longer.
Why else wouldn’t Netflix show the “continue watching” row first instead of forcing you to scroll past algorithmic generated crap?
It is the same reason that Google went from describing success as people getting off their site faster and going to one of the “ten blue links” to the shit show it is today.