You are correct that all players start off with standard racquets. However, customization usually starts because of the variances in racquet specs even in the same model. We custom weight and balance a lot on racquets for HS, college and lower ranked pro players who have 3 or more racquets.
Once they begin to feel the difference, they usually start tinkering to get that perfect feel from every frame. So you can see how the pros over time begin to customize their frames when working with the manufacturer or customization house.
Since most companies are trying to sell their racquets to the masses they cannot use pro specs as they would be too hard for rec players to play with. Their best attempt is to make a Pro, MP and light version of a model and let you select. Most companies will just say buy the frames that feels best to you and customize it to your desires.