Each device have their own application. Their application works very well. To integrate a device, We must create servers, modify firmwares etc. In reality, if there would be a way to enter all identifiant and passwords into a link in the Device Handler that would go to the original applications. It would surely require a permanent computer or a Pi3 raspberry. But by doing this we could integrate all devices.
smartapp as wood, aukey, kaza. He have good smart product with low cost.
Tp-link is integrated with node server but we don’t have the refresh of state if you changed manually but he work with Alexa
Your observation us correct… But this is the current fundamental architecture of SmartThings and hasn’t changed much since its launch 4 years ago.
They are working on ways to simplify this challenge.