Quicker to fit in a factory? I’m guessing they have a little air tool that presses the pin in rather than fiddling with a quick link by hand. Bear in mind the number of bikes that are built with cheap shimano kit (i.e. BSO level bikes) far outnumbers the number of even entry level mountainbikes I suspect being able to use common tooling is a selling point to the big factories.
9 speed Shimano chains used to be made by KMC, no idea if that still true for 10s and above, they’re apparently different, but whether that means they’ve moved production in house to new machines or if they just have exclusive use of a process at KMC I’ve no idea. But KMC chains were usually the same price and came with links rather than pins.