Both have their own justification and issues.
When in example when IWI designed the ACE for military users initial stocks though similar to Mil spec M4 stocks were a proprietary design.
So a user couldn’t swap the sock for whatever they wanted. From a commercial perspective this was awful, But from a military standpoint it makes sense as one stop shopping. Just buy spares when you buy the rifles.
Because as a military you’re buying by the hundreds if not thousands.
For the civilian market where you are buying individually the standard mil spec tube makes sense as the end user can customize to their needs. So later commercial ACE ended up with mil spec tubes.
For American or even European rifles it makes sense to use the Mil spec AR15 buffer tube as a standard design even on rifles like the M7 that don’t have a receiver extension buffer. Because you can use that to tailor the rifle or replace a broken stock. So if you want a stock with a cheek riser for example it’s cheaper to buy a module than a whole new stock.
For the PLA as the PRC doesn’t have a large civilian small arms market why would they need to have the commonality with a commercial standard? Though I am sure some company in the PRC would happily offer Mil std AR15 style stock modules, why bother? It doesn’t have the same logic. The PLA could adopt proprietary mounting systems and stocks just as it has magazine and caliber because its military is a “captive audience”.
The U.S. Army on the XM8 looked at a proprietary mounting mechanism the PCAPS. It was supposed to offer a return to zero easy attachment for scopes, sights, bipods, grips, aimers and lights. Good luck finding anything that uses it though. Because well it made sense from the Department of War perspective ultimately all the new accessories were being sourced or derived from commercial industry. So Mil Std M1913 remains the norm. Eventually you got Mlok again because the commercial standard. People don’t like the cheese grater rails, Keymod didn’t win the sales and so Mlok is being adopted. The US military could have put its thumb on the scale and skewed that by demanding PCAPS but doing so would require making an investment. Money that could be better spent elsewhere.
Yet that only makes sense because there is such a well established industry.
For China the PLA and PAP are the driver, there is no competition that isn’t integrated into the system.
For Norinco looking to sell a commercial QBZ191 derivative in Canada or Europe or Africa it makes sense to chamber 5.56x45mm to make the rifle Mlok compatible, to use AR15 style socks to accept STANAG magazines. But for The PLA they have to live by the dictates of the PLA.