A spreadsheet on it's own is a thing of technical beauty but for market domination you don't want to keep pouring resources into that one product, you want a suite of complimentary products.
You want to be able to embed your spreadsheet into a document, into a slide presentation. You want cutting and pasting to work sensibly between products. You want consistency in the menus and layouts.
Bill Gates understood all of this from the beginning, the same as he understood that the strength of a PC operating system is not how reliable, memory safe and performant it is, it's in how flashy it looks and how important the windows paradigm is.