I'm not sure about this. Besides removing obvious hate speech, which sort of content curation do you mean? I thought its success was due to everyone being able to upload and share video clips. I don't think curation was an important feature.
Maybe it wasn't in the beginning, but Youtube is a Google company now, and their entire business model is based on (mostly) algorithmic curation. The business model of every content creator depends upon catering to the preferences of that algorithm, and their success at doing so determines their value to advertisers. Entire genres of content have come into being and have been destroyed overnight because of the algorithm.
Youtube's success mostly comes from driving discovery of new content through recommended channels and videos.
I'm not sure the discovery/recommendation algorithm is the same as curation. It seems to me these are related but distinct fields.