Most parents probably couldn't even manage to parse the research literature to develop a formula of comparable quality to the commercial ones on their own (the published recipes I've seen on the internet certainly haven't even tried), much less source or prepare the necessary ingredients.

And if you haven't researched, developed, tested, and manufactured an adequate supply of your homemade formula before your kid is born, you probably aren't going to want to do it afterwards, even if you have the required skills.