This has been discussed at length in multiple threads with different people having different reasons. You can find a lot of this discussion in this thread above or in the following.
New SmartThings Alexa skill (2020)
One of the most common reasons is that people prefer to use the device manufacturer’s native integration rather than the smartthings integration because it provides more features. And because it will work even when smartthings isn’t working. Yes, you can go through the Alexa app and individually disable each of the devices but that gets to be a lot of work, particularly for people who have several hundred devices.
In my case, it’s even simpler: it could kill somebody. 
I have medical monitoring equipment which I never want exposed to any voice system, even for a minute, to prevent its accidentally being turned off.
Another community member gave a similar example. They have three locations on their account: their own home, their sister’s home, and their mother’s home. The mother has an oxygen tank plugged in in her bedroom. After the new skill arrived, the sister gave a voice command to turn off all lights, and it turned off the oxygen tank at the mother’s house as well. 
Still other people have privacy and security concerns: they just don’t want a list of all their devices showing up in their Amazon or Google Home account.
The main point is that this was an unadvertised change, and there was no way to go back. For something with potentially fatal outcomes, that’s pretty serious.
So from a convenience standpoint, the duplicated devices probably affects the most people. But from a feature design standpoint, killing even one of your customers is bad business. Just sayin’… 