The 2 year stipulation is/was designed as incentive for Government Employees or Former Servicemen to get on with Contractors. The idea being that they are more familiar, typically more trusted having come directly from Federal/Military environment and trained up to Military/Government specifications. The 24 month or 2 year stipulation is for a renewal window, of sorts. If you are within the 24 month window, the process by which you can get your clearance renewed is easier and faster. The term fast tracked is exactly right in situations such as that. That is not to say that a background check or the process of filling for a clearance is not necessary, it is. However, the 2 year window makes it easier. If you are outside of that 2 year window, then the process is much more expensive and it takes much longer.
That is why it is recommended that you keep up your clearance, even if you are not employed by somebody who is paying for it. The 24 month window, in no way, precludes you from having to get re-certified for a clearance. You still have to do that. Once you leave the Military, your clearance is revoked and you have 2 years to apply for recertification. After that 2 year period, you are viewed as a risk level that is comparative to any other civilian who has no clearance. You will then get put into that pool and the process for clearance (unless you are highly specialized and your application has significant push behind it) is very lengthy. The waiting list for applicants for clearance outside of the 2 year period is really long. We are talking anywhere from 6 months to a year and even longer depending on where and why. If you are in the job market and you have to wait that long, chances are the job will not be there for you. Somebody else will have been hired and you will be out of luck.
That is why the 2 year window is important but, in no way does it mean that you do not have to reapply for a clearance or that you bypass the process associated, including the background check.
Now, I think that we are done here. I have explained the process as clearly as I can. You can either accept it or not.