Typewriters. Not the 1980s typewriters that were half computers, half type machines. The double space habit comes from old manual typewriters, which employed monospace fonts. In comparison, fonts used nowadays like Helvetica and Times New Roman are proportionally spaced. This means that the letter i gets less space than the letter m. In monospace, both get the same amount of spacing (due to the moving mechanics of an old typewriter). As such, there would be a little space around an i to compensate for the larger m. Thus, double spacing after periods was used to make the end of a sentence more obvious. Of course, that was over thirty years ago. Today we only use monospace fonts if we want to create the good ol' feeling of typewriters, which most people of my generation have never even touched. So why do people still do this? Either they want to act older than their age, or they never got the memo that double spacing is wholly unnecessary. And, in practice, for every person who double spaces consistently after a period, there are at least two who, somewhere in their twelve page paper, forget to double space once. Just once. And that is inconsistent and ugly.
And if it's hard acclimating to single spaces, use Word's find and replace feature. There should be no excuse to using a single space after a period.