Because they're not paying you in bitcoin or by brown paper envelope. There's federal, state & local city tax reporting for each employee, W-4's to fill, W-2's to mail, health care providers to setup (different in each state even within the same company), 401Ks etc. Unless you're going underground not only from your employer but also the tax authorities.

Now if you maintain your high-cost address but go tripping around the world, perhaps they wouldn't find out for a while.