Don't want to stir up any heated debate again but I do have some insights to share if anyone are serious about getting into audio professional/commercial business.
Any core boards should be as close to your own business as possible (design files, bill of materials etc). You should need to be able to switch to any PCB manufacturer at any moment for any reason.
If you have bought loads of periheral parts to have in stock (i e header cable with connectors/jacks as suggested to be of use in the connectivity of these boards in this thread), you'd easily be in deep trouble if you are only in second position in sourcing the core boards that you are planning to connect with your peripheral parts. What do you do if you have customers waiting to buying your products but your original core board supplier has withdrawn their ability to deliver for any reason and you don't have any design files etc?
Secondly, who would be responsible to handle any warranty of your products if there are a core board that you have pretty much no control over (any design alteration may affect your product capabilities drastically)?
Any type of connector/jack with header cable is quite an manual labour in production. Which unfortunately usually means expensive to buy unless you buy a huge amount from a supplier. There are quite some options out there regarding connectors/jacks to choose from. And what lengths should the header cables be provided in?
Big professional brands may/often use such solution between core, ui and i/o boards. Some of the reasons are ease of manufacturing, further development and/or alteration in any of those boards area. As well as ease of fault test/repair.
I haven't seen it myself regarding the connector/jacks themselves having a header cable at their end. But they probably exist somewhere in some product out there. Point is that the costs probably speaks against it unless it is a niche product. Which probably is why you/me/we don't see it more often in commercial products.
In production/assembly you often see either an i/o board solution (header - header) with specific configuration of connectors/jacks OR a core board with specific configuration of connectors/jacks.