O.K its not rocket science and a good 2 day training course will teach you the basic's, the trouble it that there are plenty of untrained carpet cleaners out there who will happily use dangerous chemicals in peoples house, soak carpets which can cause mould and mildew problems (potential health hazard) and attempt to clean carpets not suitable for wet extraction , Belgium Wilton's and Seagrass / Sisal to name a few, clean the last three and you risk major shrinkage. Then you get into upholstery which can be a minefield. Cleaning carpets can be easy but getting stains out is a whole different ball game that a lot of carpet cleaners don't even bother to attempt. Then you have health hazards to the carpet cleaner, like needles down the crevices of sofa's (H.I.V risk), blood, feces and urine can also be a major health hazard. What about flood damage where there is also sewage contamination, doesn't sound quite so easy now does it?
Trouble is that there a lot of people who think it is easy and so don't bother with training and end up causing all sorts of problems, some of them fatal.