Just to note something - the clothes and often robe like materials worn by Gurus usually has nothing to do with spirituality itself, but is traditionally due to the fact that such clothing is the most comfortable to meditate in for extended periods of time. Likewise in certain cases robes are worn simply because they are cheaper than modern clothes, and acts as a sign of modesty - however obviously not all Gurus follow that modesty aspect, as some such clothing are extremely extravagant looking.
Suits / coatpants are usually far less comfortable than any kind of robe or casual material, as a general rule - due to how they often constrain motion in some way, even if tailored. Plus, also remember that "western culture" didn't really exist during times when most authoritative texts were written. Even modern "shirts" don't really go back that far.