Absolutely agree- however, in many cases these types of companies can still be decent businesses. Being able to work with and adjust "moderately complex" Excel sheets requires a certain level of training and skill- often these startups can remove the requirement for expertise to perform a task, which can be worth quite a bit to a business.
The biggest feature of Excel is its flexibility. That moderately complicated spreadsheet can and does change all the time, to accomodate new requirements and optimize workflow. A startup (or the internal IT department) doing one-size-fits-all solution can't really do that.
This is probably its biggest flaw also — chances are you don't need all that flexibility within a scope of particular spreadsheet; and sometime you may prefer a solution that was specifically targetting this exact usecase.
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.
Many, many startups are competing against a moderately-complex Excel sheet, and they don't even know that.