I recently learned that a fully qualified domain name ends in a dot, e.g. www.microsoft.com.. Why do I almost never see this? We casually toss around www.microsoft.com, a relative domain name, without a second thought.

It's not mere pedantry, because it seems there's a security issue: someone in your network could create www.microsoft.com.example.com. to hijack your requests to www.microsoft.com. Why doesn't the internet community have a tradition of using genuinely fully qualified domain names?