Wouldn't they just use a 3rd party payment processor these days (e.g. stripe or similar)? I can't imagine taxes, chargebacks and other things being such a huge issue now for a small independent who could likely leverage one of the many available payment platforms.

Marketing seems to be the biggest draw for using a publisher these days. That and getting easy access to partner management with sony and microsoft. My understanding is that some publishers are even quite good at social media now, helping indies get exposure on twitch and youtube.