How is it that Rooting is always possible? Having spent some time researching what actually goes behind rooting an Android phone, the main reason that leads to the ability root is an inherent vulnerability on the Linux kernel that leads to the exploit and getting the 'su' binary to be installed. 

My question is: How come the OS is not hardened yet? Who is responsible? This is a Linux OS we're talking about, which is considered "secure". Yet, people always find a way to get in and root devices.
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.