@rumtscho answered the "why" for the title question very well.
Here i will try to give "the how" to solve the problem...
Am I doing something wrong? Or is that the consistency I can expect from a sour cream whip?
You could modify your procedure to something more similar to making a "Diplomat Cream", which is custard mixed with whipped cream.
- Mix the salt, sugar, sour cream and bourbon. This may involve waiting for the sugar to dissolve into the sour cream (avoid heating because sour cream will split if overheated). Then refrigerate until needed.
- Whip the cream alone (over an ice bath) to medium peaks. (This is the point at which the tip of the peaks curl over on themselves when the beaters are lifted).
- Ensuring that the whipped cream is still cold, gently fold through the sour cream mixture. This would be best achieved by sliding a thin edged scraper down the side of the bowl, and lifting the whipped cream through the sour cream.
It should also be noted that the stiffer the whipped cream is, then the more it will resist mixing and may hold in clumps, plus you risk overworking the cream (when trying to smooth out the lumps) which leads to splitting. So err on the side of caution when whipping the cream. It is better to sacrifice a little lift/aeration than to ruin the batch entirely.