You've just answered your own question. It makes no sense for a company to hire "top-notch developers" to solve "the problem" that storage vendors have already solved (because they have truly top-notch developers with vast experience in this specific domain).
That would turn out to be way more expensive than just paying what you consider exorbitant prices, and way more risky, i.e., it's not like it's easy to find top-notch developers, and even if a company could hire some, it's very unlikely that whatever they developed would compare well, by any metric, including cost, to what the company could get by buying a ready-made solution from EMC, HPE, et al.
That would turn out to be way more expensive than just paying what you consider exorbitant prices, and way more risky, i.e., it's not like it's easy to find top-notch developers, and even if a company could hire some, it's very unlikely that whatever they developed would compare well, by any metric, including cost, to what the company could get by buying a ready-made solution from EMC, HPE, et al.