Because adding dirt to your cartridge is not good for the bore of your rifle.
The secondary explosive effect takes place with slow powders, IMR 4831 or N205, when used in low case fill loads.
This effect is not known to happen with faster powders such as IMR 4064 or H4895.
Choose an appropriate powder for the cartridge and bullet weight you are reloading, and all will be well. No need for a case filler.