I mean, they mostly want to run code third parties wrote not themselves and not just what the manufacturer decided to write. As you see in the router example someone game. As you see in the giant example of android/ios ("There's an app for that"). Even when there isn't an appstore like with desktop operating systems. Even when it's a pain in the ass to install third party apps like remarkable tablets. Even when the "thing" isn't hardware but a base layer of software like minecraft.

In fact, I can't think of a single example of a device that was popular, made it legal and not overly difficult to write and distribute third party code, and didn't have a thriving ecosystem.