It probably doesn't help, but it definitely isn't the reason. Most people still do their work on desktops with full-sized keyboards and they already don't use those keys there.

So how they e.g. move around cells in the spreadsheet or documents?

Mouse. If you're correcting data in a spreadsheet you're probably jumping around a lot and a mouse makes more sense and if you're entering new data most people I've worked with use tab and enter.