Why do you find that it makes so much sense?
I've never had a problem with them being separate programs. In fact, it makes it easier for me to configure everything, since the preferences are discrete, not all lumped together. Plus, they mesh together seamlessly, so it's not cumbersome to have them separate.
Because I may want to use the calender without loading my e-mail client or I might want to send an e-mail pronto and not have to load a bloated program like Outlook. It makes more sense to keep them separate rather than lumping them all together is some huge god awful piece of software like Outlook.