Because you might not don't trust a company to do a good job at the price point in question.

In the past when I was using legal prohormones and needed to feed in a lot of protein and nutrients, a 1 gallon milk jug filled with mix and stored in the fridge was ridiculously efficient. There's no way it was more unpleasant than Soylent; I distinctly remember kind of enjoying the chocolate flavor.

The deal is that people have to know what their body needs first and foremost. Gain? Lose? Maintain? All are different goals and methods. Solyent advertising as an off-the-shelf works-for-all-people might be fundamentally true, but I don't think it can be argued that it's the ideal for each and every person.

That's why I started the sentence with "if you care about maximizing the quality of your product" -- it's a trade off. If you are ok with getting the average stuff, it's absolutely not necessary to customize your own MRP. The problem is, in the supplement industry, "average" often times means bad, so at least do a little research on any off-the-shelf supplement you're about to buy.