My guess is the system creator think a lot about making the best recommendations for situations where they have enough information but seldom think much about the consequences of making recommendations where they don't have the information to make a recommendation and both users and developers underestimate how information is needed for a decent recommendation.

Edit: And the situation seems very much a product of youtube (and all the content providers, basically) stripping any user content controls from their UI. The user has no easy way to tell youtube what they want besides choosing between video that youtube offers them and those become more circumscribed as the process progresses.

There's a myth of the content providers doing magic to give the user what they want and the results seem to be this, ie, just terrible.