The main reason is K factor. One may not know as what value of the K factor fabricator would use. Now if you give the details in flat pattern view taking K factor as 0.5T but fabricator used 0.38T as K factor. This would give the flat pattern matching with the drawing but the bend part dimensions may change. So people tends to give details in bend form (as that is the final position) and let the fabricator calculate the required flat pattern.
That makes sense, thanks for making me understand this practical issue. This question had been chasing me since a long time.
Have a great day ahead!!
Thanks once again.