Probably the same reason in many sports like cycling white is the winning color. It gets dirty so easily that you can't really keep wearing it. A tie is out there touching and getting touched. It will likely have a shorter life. Plus the modern bright white is fake and involves additives that shift higher non visible frequency wavelengths into the visible spectrum. That's why your white towels and bright white undershirts turn less bright and more of an ivory over time. The additives needed to create that bright white may not work with a finer silk thread. Everything I can think of with really bright white are a heavier weight thread. That is just off the top of my head. I think I learned the white additive thing from Veritasium on YT years ago. Edit: no it was "Nighthawk in Light" on YT that I learned the white thing from.