Why do we need experimental data? I am an engineer in a data science company working on price optimisation.

Every year we look at the historical data, try to predict people's elasticities and based on the them we set the prices. The historical data is mostly observational. 

What I don't get is why our economists want to run price experiments each year, I.e. give a certain group our calculated price plus or minus x%?! Isn't observational data very good for our modeling task?
By explicitly setting the prices we somehow run experiments, don't we? Moreover, separating people into "randomized" groups afterwards to measure the average treatment effects, should be possible in my eyes. 

Many thanks in advance!