Septic tank concrete flooring
A septic tanks purpose is to allow the anaerobic bacteria to break the effluent down and also allow the aerobic bacteria to die.  This takes time so a septic tanked should be sized based on the inflow to allow a slow enough change over (in days) for this action to happen.  Then, in the drain field, the anaerobic bacteria dies off when it meets the soil and aerates.

For this to process to happen septic tanks also need to be completely sealed; this is critical otherwise the effluent does not spend enough time in the tank for the bacteria to work or die. 

Concrete septic tanks are not simply concrete but concrete with a coating to make them impermeable to water because concrete by itself allows water through it.  This means effluent and its aerobic bacteria could immediately enter the soil before it had a chance to die off and continue to possibly thrive.

Further if you had rocks at the bottom you would not have a septic tank you would have a cess pool.  These are a health hazard due to the fact the aerobic bacteria is not "dealt" with all and can proliferate through the soil into ground water and such.

Point is a spectic tank should completely contain the effluent; no leakage at all.  This is why sealing cracks if found after time in tanks is important as well.