Because labels run properly have great data to market more effectively, and can spin up tried and trusted playbooks, because they tend to be genre specialists. An electronic artist won't sign to a folk label - but a folk artist signing to that label can plug into a whole bunch of proven strategies, which mean far less money and opportunity is wasted. There's a whole bunch of stuff - pixel data, audience profiling, relationships with playlist curators, email lists - that a label can apply to an artist they sign.

Releasing music onto YouTube or Tiktok is not marketing - it's distribution. Distribution is trivial, it's a commodity service. Because anyone can distribute music, there's lots of it. Getting much heard requires specialist expertise - and that's why there's probably more value in good labels today than there was ten years ago.