Well because it's a huge deal in my opinion. If someone steals my phone they can't get back into it even if they wipe it.
If they know how to root it, they can in 2 minutes. Just boot into the bootloader, flash a custom recovery, flash a custom rom, and presto, you're in. If you're already rooted, well, that just makes it laughably easy. Apple's approach is the only really secure one.
I think assuming that a thief would have any understanding of rooting or flashing custom bootloaders would be a little over the top. Out of every single human being that I know only 2 know how to root a phone.
XDA Developers was founded by developers, for developers. It is now a valuable resource for people who want to make the most of their mobile devices, from customizing the look and feel to adding new functionality. Are you a developer?