Discovery is fundamentally hard because its success is judged by two parties whose interests align on paper -- for publishers to find a receptive audience, and for a consumer to find enjoyable content -- but the market is so vast that a missed match isn't just that, but often a match for a different pair. And consumer attention is vast, but ultimately finite, in that it suffers from oversaturation and requires novel and exciting experiences to stand out from a sea of similar offerings.
So platforms use recommendation algorithms to try and guess the sort of stuff every person likes, and try to surface content they're more likely to appreciate, instead of surfacing something completely random. Content that's brand new has no data on its audience, so a chicken-or-egg problem occurs where popular content can bubble toward its most fitting audience, often growing until it can't, while never-before-seen content languishes in a popularity vacuum, desperate for out-of-band help.
The classic out-of-band help is word of mouth. Sponsored, promoted content is a kind of in-band help, allowing platforms to make money and producers to jumpstart their exposure, but has the same structural properties as popularity: those most equipped have the potential to rise the most, while those least-equipped are disadvantaged once more.