Hello!
Because there are real scientists using the real scientific method who were unable to reproduce even the smallest effect.
There is at least _some_ scientifically gathered evidence that magnetic fields can alter the crystallisation of calcium carbonate. Whether this leads to less calcium deposits in water pipes has yet to be proven, though. When we moved house last time (10 years ago) I thoroughly researched the subject (with the aid of a physicist) and came to the conclusion _not_ to spend any money on magnetic water treatment.
Regarding magnetic fuel treatment, there exists zero scientific evidence and zero scientific explanation of the effects claimed.
Here is some reading stuff for those interested in the subject:
Quick overview:
CSI | Magnetic Water and Fuel Treatment: Myth, Magic, or Mainstream Science?
In depth literature research with many links:
Magnetic water treatment pseudoscience
Regards,
Max