The flipside of this is that you can open an Excel file and expect a full VBA application to begin running immediately.

Or well, luckily someone eventually realised that that wasn't a good idea so now they just open with a big warning saying that the file in read only mode. Hopefully you also get some more intelligible warnings when it actually tries to run VB code.

Perhaps, but I think it's more likely that the only feasible option was to use VBA with excel, and excel is the "default" spreadsheet application used everywhere. It nearly the same reason why JavaScript became the default front end language thanks to Netscape, which was the "excel of browsers" during the dawn of the internet.