As a kid, this didn't make sense. Doesn't soap clean? Why would it make the carpet get dirtier faster then?
How Soaps Work
Soaps if not rinsed out of carpet can become attractants to dirt and grime. The science behind soaps are powerful but also simple. We use soap every day of our life to wash our hands, dishes, and clothes.
All soaps have two parts, a head and a tail. The head is commonly referred to as being hydrophilic or water loving. The tail is lipophilic or oil loving. Because of this unique combination, the tails are attracted to oil and will surround and create a thin layered coating around oil particles. When surrounded, the oil will wash out being carried away.
Soap Residue in Carpet Gets Dirtier Faster
Soaps as a Residue
If there is soap residue left on the carpet from steam cleaning then the oil sticks to the lipophilic tail and then becomes a binding point for additional soil particles.