BecomeMe

Actually YouTube's revenue share model is widely regarded as the most generous of all mass content distribution platforms. YouTube Partners not only control how many ads are served for their videos, but make 55% of ad revenue generated by their videos. Your observation about "notoriously low" ad revenue is actually showing that the total ad revenue for YouTube as a platform is low. This is why more ads are being served now, because the price that advertisers are willing to pay for placement on YouTube (and across display ads generally) has gone down over the past few years. As a result, creators choose to max out the number of ads served on their content. Part of the drop in value of ads is attributable to the relatively high value target audience of sophisticated, tech literate, generally younger viewers who use adblockers removing themselves from the market. YouTube is telling this group "pay up for premium, or opt in to ads" in order to convince advertisers to pay higher per-ad prices.

To @bananatriffleviolin's point below, this hopefully will drive a shift back towards pay for service models for creator submitted content at large. YouTube Premium subscriber views pay creators way better than ad-revenue from the same video viewed by others for example.