DeX is a flagship feature restricted only to Samsung’s flagship ranges only. Samsung have curated and market their mobile devices based on the Individuals uses & need.
Lets assume that Samsung started giving this feature in all of the mid range devices then there will be no difference between the Flagship & mid range budget phone. If user want that feature he will obviously buy the mid range phone in which that particular feature is available, rather than the buying expensive flagship devices.
By using 3rd party app we can do that but it is not as grt as the inbuilt feature. And majority of the market is in mid range only so they have to provide at least some of the features.
