Yeah, but a programming language would be even more flexible, and yet nobody says they don't need software because they have all the tools to implement its functionality themselves.
In fact, I worked on a project that largely aimed to replace a bunch of internal spreadsheets because giving groups a spreadsheet to maintain is a little like handing Phaeton the reins to Apollo's chariot.
Sure a programming language would be even more flexible, but it does not have an instant feedback loop like Excel does. Not to mention that the extra flexibility comes with the cost of dealing with all the arcane bullshit programming involves - toolchains, APIs, what have you. Excel is flexible enough to be useful by non-tech people and doesn't bring in so much cruft.
In fact, I worked on a project that largely aimed to replace a bunch of internal spreadsheets because giving groups a spreadsheet to maintain is a little like handing Phaeton the reins to Apollo's chariot.