[stg22]

Social media sites have so many users/viewings that fixed costs for serving all their content don't really matter to them, but since ads don't pay much per viewing, they have to cut the marginal costs of hosting and serving content to the bone to stay profitable.

For every million pieces of user-generated content hosted, they get a certain number of copyright complaints, so handling those complaints is a marginal cost. Since the kind of uploaded content YouTube target is particularly prone to being a significant violation of copyright (e.g. entire films), handling the complaints cheaply is a particularly big concern for them. Handling them effectively and with common sense would require human processing of each complaint, but the effect of that on marginal costs wouldn't fit with the business model, so they go with a low-marginal cost, error-prone system instead.

Since the consequences for the business of wrongly failing to take down copyrighted content can be very bad, but the consequences for the business of wrongly taking down uncopyrighted content are negligible, if the system must be error prone, they want their users to be on the wrong side of any errors, not the complainants.

Any American rival to YouTube would have the same problem and would resolve it a similar way.