In many fields, spreadsheet apps still are the killer app(s). I worked at a company that made tax software, and to this day Excel is still a mandatory skill for an accountant, despite the "advanced" software we have these days. In project management in many industries as well the data is largely spreadsheet driven.

Couple that with the fact that you can literally use a Google Sheet as the database backend for small apps, and I think it's a good case that spreadsheets are still killer.