How is it that Rooting is always possible?
it's not really the "linux OS", it is the linux kernel. The vulnerability is not necessarily in the OS, or the kernel. There are different exploits that are used. I remember some device being able to load any zip file in the recovery if it was "pre-pended" by a signed zip file. So they used a signed zip and added an unsigned zip to the end of the file. I think this exploit was for the Original Droid.

No software is 100% bug free. if it was, then even a mature product like Windows would not have to do security patch releases every month.

No matter how smart the developer is, or thinks he is, more likely than not, there is probably someone smarter (or at least they may know more about exploiting code).

When exploits are discovered, Google (or the manufacturer) has (usually) patched them. But since a lot of the devices don't receive updates very often, the exploit remains available.