Not only that, but I don't trust myself to remember how I calibrated myself to the rating system in the past. If I'm more stingy with the 5-star ratings now than I was 2 years ago, then I'm going to skew things in a way that the system probably can't understand.
In other words, it not only expects me to define my own scale, it also expects me to stay consistent with it over time. I don't think that's realistic.
I honestly wouldn't mind if from time to time, a service asked me to stack-rank movies. Because that would be a question I feel confident I can answer. Give me two movies and ask me to say I prefer A, prefer B, or don't have a clear preference. Or give me 5 movies and have me put them order from best to worst, allowing me to say two of them tied or exclude some. Maybe the UI on this would be too weird for average users, though.