If Sony own Death Stranding, having 505 Games publish on it Steam makes literally no sense. Sure, commission them to do the port, but why have them publish as well when
Sony has had a publishing arm on Steam since 2015? ¯\_(ツ)_/¯
Late to this discussion, but I feel these two are tangential.
Ownership of the IP and who publishes it does not have to be the same.
For the question as to why Sony did not want to publish it and asked 505 to publish it, it may have come down to strictly just experience and available resources. Sony did not normally distribute games on PC back in 2019, so they may have at the time wanted to go hands off and see what was possible for DS, ie; they didn't have the resources to do it.
It's not so straight forward publishing on PC. The publisher will provide Q&A for the game on a variety of devices, some things need to be re-written for steam, like leaderboards, separate online server infrastructure etc. If it's a cost Sony didn't want to waste their effort on, then they won't. Someone has to pay to put in additional settings, features, infrastructure, switching to DX12, needing API support, and testing for PC and hardware vendors etc. These are all things the publisher will support the developer on, if you have DX12 issues, you don't go directly to MS. And so, in some fashion, Sony may not have had the ability to support KojiPro in this fashion in 2019.
There is a specific reason why Sony now has a new publishing arm called Playstation PC LLC. It's not just a name. It's a entire division dedicated to supporting their PC releases; they likely didn't have this with Death Stranding.
So I just wanted to put that out there as to why they went with 505 but they likely will no longer go with them. I think this is in the same vein that MS after purchasing a great deal of many companies, did not take over at Publisher for the games already inflight (not to mention publisher rights..). The work and relationships, tools, and process flows between publisher and developer were already established, so they were hands off and only are publishing their new titles.
Death Stranding IIRC is owned by Sony however on an aside.