Lets start by asking you some questions:
Why do we wash our hands? To get rid of harmful bacteria and viruses.
Why do we wash our clothes? To get rid of harmful bacteria and viruses.
Why do we shower and bath? To get rid of harmful bacteria and viruses (and to smell better).
So why isn’t it a good idea to allow sewage from the septic tank to enter a ditch, creek, lake, water table, etc? Because the sewage is full of harmful bacteria and viruses.
Often times I quote the story of children getting sick because the family pet was outside drinking the water in the ditch that had a sewage discharge and would lick the children which transferred the harmful bacteria and viruses to them. This is just one scenario of how easily the harmful bacteria and viruses can enter into our bodies if untreated sewage is allowed to discharge on the surface of the ground or into a water source. A large scale scenario is where a small community has contaminated drinking water because a majority of the septic systems are to close to the water table and/or are failing due to other reasons.
Sewage needs to be properly treated so the harmful bacteria and viruses are removed before entering the water table or other bodies of water.
Oh, and the other reason why you can’t is because it’s illegal.