Why? Can anyone answer who knows the reason
It defies logic
I don't know with 100% certainty why it's common practice, but I'm pretty confident of understanding why it's done.
It's expediency.
We need to remember that most BIR's and T/A's are working on a time/cost/profit basis, in other word's they're being run as a business to produce profit and they will always take any shortcuts or cut any corners they can whilst still producing an acceptable end product. Why buy in and keep in stock all the minor ingredients found in most commercial curry powders when you can just dollop in a couple of tablespoons of commercial curry powder and the problem is solved? It's quicker and cheaper to do it that way - and that's why they do it.
But it's also one of the reasons that most if not all BIR's and T/A's will produce a standard 'flavour' that pervades most of the dishes that originate from that particular BIR or T/A.
As home cooks, we are not commercially constrained in the same way as BIR's run for profits are and there's no reason whatsoever why we should be following this practice at all. Particularly when you consider most of us will have in stock all the minor ingredients usually found in Commercial Curry Powder anyway.
It's interesting that NagaMark comes from the same background as I do - traditional, authentic Indian cuisine, where generic curry powders simply aren't used - and asks precisely the same question I did when I first arrived here. If we can produce great tasting (albeit not in the BIR style) Indian dishes without using generic curry powders, why suddenly in trying to replicate BIR dishes do we have to use it?
In my opinion (and it is only my opinion, anyone else is perfectly free to disagree with it) the BIR flavour is produced via cooking technique as long as the correct spices are used in conjunction with it. The use of generic commercial curry powders and/or garam masala is yet another dogmatic 'red herring'.