As a former supps company owner it's all to do with buying power, blending and formulation.
An individual buying each of the ingredients could not afford to do so. Even the smallest amount would need to be bought in the multiples of 100's if not 1000's. Companies which produce these sorts of products would want orders amounting to a 1000+ tubs (as a minimum). In reality it'd be $10,000+ to get the cost down to sensible levels (with a margin for distributors etc etc).
Like I said - pick three of the more useful ingredients - now go look at what they cost to buy for a decent price. Now multiply it by all the other ingredients. You'd have 100's of bucks worth of powders and you'd still need to make up a batch for you to consume for yourself.
It's doable for some single source products. I've done it (as have many) with Creatine. Also with Beta-Alanine, DAA etc. But I've never tried to make up my own pre-workout with, say, 20 ingredients. When I owned a supps company I was a bulk seller (5kg of protein for example)