Technically a properly functioning tank should never need to be pumped. Im amazed at these rules and regulations that force you to have the tank pumped. When I lived with my parents they had it pumped out twice from the time I was born till they sold it ~25 years later. Once was b/c we had to replaced the lines going to the tank and the other was b/c pops wanted to.
I imagine where I'm from a lot of people still straight pipe to the creek....
Did your parents place have a septic tank/leeching field, or did they have a cesspool system?
Where I grew up we had a cesspool system, which is different than a traditional septic system. I don't ever remember my dad having our cesspool pumped out until all the houses on our street were switched over to the newly installed city water and sewer system.
Everyone who had a cesspool or a more modern septic system had to have their systems pumped out, the cesspools filled in and the septic system tanks removed and the holes filled in.
I think that a "traditional" septic system, if working properly, would not need to have any type of product with yeast in it added to it if it was a perfectly working system. But as others have said, with all the things that get flushed or put down the drain that are not meant to be there, over time, even if the "****" was eaten away by the good bacteria, the other things that are not biodegradable would remain, and need to be pumped out before they got loose and made their way into the leeching field pipes and clogged everything up. That would be very expensive to replace versus pumping out your septic tank(s) every so often.
I look at it just like I do any type of insurance you have on your car or house or your own health insurance.
You may never need it, but when you do, it's a lot cheaper on your wallet than if you didn't have it.
Jim