Parties are entirely implemented via platform-specific services. So when you party up with your friend on PS4, all the data that goes back and forth to make parties work - "This guy joined our party. We're joining this server." etc. goes through Sony's system.
There's no way to send that data to a Steam user because Sony's system does not know Steam exists, much less who your friends are on it.
The only way to do cross-platform parties is to have your own independent friends list and party system backend that handles it all separately from Sony/Steam/etc. Unfortunately this is a big system and as an indie developer we have to pick our battles on what we spend our time and resources on. We weren't even sure cross-play would be approved until very recently, so it would have been a bad decision to invest heavily in a multiplatform party system only to be told it wasn't approved. It would also be an additional risk at launch for us because if our party servers couldn't handle the load, it could break everyone's ability to party up and play together, not just cross-platform, whereas relying on Sony and Steam services is a much safer and more reliable choice.
We realize that it's still disappointing and are looking into workarounds as well as considering whether we can take on a full party/friends system overhaul to allow cross-plat partying.