All our fittings are conducted on a range using premium golf balls.
Ball Flight doesn't lie. Fitting indoors you can't determine ball flight and how it is affected by different fitting options.
Ball flight and club/swing measurement gets the most accurate readings when it can see the entire flight of the ball.
Indoors, even with an optimal setup, many launch monitors have to calculate many of the numbers it reports rather than actually read ball behavior.
Lots of other advantages including access to grass when needed but those are the two biggies.