It's not that it's unmaintainable it's that it's a lot more difficult to maintain.

You can get around this be having someone engineer a spreadsheet using all of the advanced functionality and VBA that Excel offers, but by that point it becomes a lot easier to just use an application that's built for whatever purpose you're trying to achieve.

Spreadsheets are accessible for a lot of people, but that doesn't mean they're easy to use and maintain. "ease of use" implies "use" for some purpose.

A lot of time ease of accessibility (anyone can click an icon and start entering numbers and doing calcs) conflicts with actually achieving whatever your purpose is. And if often conflicts with maintainability.

Spreadsheets ease of accessibility is both their biggest benefit and their biggest drawback.