Because other people are fucking morons and their editor doesn’t have visible whitespace enabled - or it does but they don’t give a shit.
Therefore these fucking morons have anywhere between 2 and 8 spaces-per-tab configured and will happily mash the tab key however many times is convenient for them to align their code or comments because they don’t understand shit about fuck when it comes to alignement (or they don’t care). Now I open their file and everything is predictably misaligned. Spaces and tabs are mixed from one line to the next, and in particularly egregious cases no tab width I can locally set on the file will make it readable because multiple different morons used different tab widths to align with tabs - sometimes within the same goddamn function or comment.
Have you ever tried to read an important technical diagram in ASCII art aligned with tabs by different people with different IDE settings? Because I have. Emphasis on tried.