It is all for the entity known as bike forums. More repeated content to drag up weekly.
Reasons to name a few:
1) Vanity
2) It looks better
3) Easier to apply sunscreen
4) Massages feel better
5) Easier to clean out road rash.
6) Spandex with hairs sticking out of them looks awful.
I really do not understand why this question comes up so often. Many other sports involve shaving various body parts. Swimmers, body builders, etc.
Do forums devoted to these sports suffer from the same incessant questions?