Because according to Postini's last communication intelligence report, about 94 percent of all email is spam. Spam increased 147 percent in 2006 and caused a 334 percent increase in processing requirements for corporate email. Some systems are simply melting down under the pressure - too many messages for the medium to handle. And when you figure in the massive amount of hardware and energy to keep the global electronic mail system afloat, a six percent 'success rate' is just unacceptable.
There's a lot of talk about how to solve the problem. Phil Wainewright from ZdNet thinks that, from the cost perspective alone, running an in-house messaging program borders on obscene. George Ou from same thinks it's not that bad, and of course there is always Google who has had both feet in the game with GMail, one of the largest email providers in the world.
Out of these options, services like GMail will probably carry the day. Centralization reduces the massive redundancy of resources - multiple email servers, support personnel, etc. - and it also reduces spam, as each one that is detected is removed for millions of users at once. That's a lot more efficient than having thousands of email servers getting sent the same spam, checking it, then having each one delete it for a few hundred users. In sum, global, centralized email is probably the best way to deliver that service. And that takes into account people, profit and the planet.