Except that's not what the algorithm does at all. It shows you things SIMILAR to what you've already watched, but not exactly the same. Of course no one wants to listen to the same songs over and over, but Spotify recommends you SIMILAR music so that you have a chance of finding something else that you like.
Sure, if your goal is to broaden your horizons and find media that's different to what you normally consume, then those algorithms probably won't allow you to do that. But for Youtube and Spotify, it means their users are constantly finding more things on their platform that they enjoy, which means more time on the platform and ultimately more ad revenue (or revenue from other premium services they offer).