It isn't always, but when it is, it's because the costs outweigh the benefits. Usually this is in the form of lack of employee flexibility to do the best for the company (i.e. red tape and hoops become an impediment). However, in this case the cost is employee privacy. You might argue that they have none and spying on all traffic is reasonable, but many times these policies encourage employees to use workarounds that are even more dangerous just so they can have a modicum of privacy (e.g. alternative and less-vetted software packages, non-company hardware, etc).