This is overlooking an incredibly important piece - composition and reliability. Infants, particularly those under 6 months, have very specific nutritional needs. A subset of that population has extremely strict nutritional needs. Even in a health baby, "stretching" the formula with too much water can cause a dangerous drop in electrolyte levels.
For the average person, it's going to be extremely difficult to source, process, mix, and hydrate ingredients (and test it all) to properly feed their baby.
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To put things in perspective, toddler formula is _extremely_ similar to infant formula, but the ratios are slightly different. This slightly difference means you cannot safely use toddler formula as a replacement for infant formula.