National Small Flows Clearinghouse, which is a national environmental and wastewater industry publication, had this to say about a question they received about "septic tank additives"to their technical assistance hotline:

"...Often, manufacturers of biological additives market their use to restore the bacterial balance in a septic tank on a monthly basis as part of a routine maintenance program. This is not necessary because these bacteria already reside in human feces.

There are special instances when the use of a biological additive may be warranted, such as when homeowners take antibiotics or other prescription medications. However, it should be noted that research is still needed in this area." 

You can read the entire article by clicking this link to the Small Flows Q&A article.

As mentioned before on this website, you will find documentation about the affect of septic system additives at the InspectAPedia.com website.

Both of these sources do a pretty good job of explaining "Why" additives simply don't work for the long haul or simply don't work.  The fact is, it is the nature of anearobic bacteria that causes a system to fail.  It's simply a matter of time.  By the time a septic system is 20 years old, the drainfield has done as much as it is capable of doing.  It simply can't do anymore.  However, with the introduction of oxygenated effluent into the leech bed, fast growing aerobic bacteria can take most of those years off of the system.  That's just the way those things work.  It eats so fast and furious and so completely that it literally "cleans" up all that goo!