It's because at some point, your code will encounter low-tech technology that doesn't let you set the tabstop. This might be a terminal app, or an actual dumb terminal, or a cut & paste into a textbox on a webpage, or a third-party tool that expects spaces only, or a refactoring tool that you wrote yourself but don't have time to get working with tabs, or a parser that can count characters but has no logic for tabs. When this happens, your tab-based formatting will get messed up, while your spaces-based formatting will look exactly the way you wrote it.

A lot of mysteries about why people don't use clearly-superior technological solutions are solved by understanding that ubiquity is a feature, in many cases the most important feature. I don't use vim because it's the best text editor; I use it because I can be reasonably sure every single UNIX-like system I ever log into will have it, and once it's in my fingers from having to learn it on a remote server, I might as well use it for daily programming. I don't use HTTP because it's an efficient protocol; I use it because every single device, library, and language speaks it. I didn't write Gumbo [1] in C because I like the language; I did it because every modern language can bind to C libraries, and so this lets a maximum number of people use it.

[1] https://github.com/google/gumbo-parser