Because the increase would create more of a deterrent and improve society .
I think that's the main issue. Tougher sentencing doesn't tend to reduce incidence of a crime, or at least it's rare that it does. I appreciate that it's counterintuitive.
However, increased likelihood to be caught for a crime does reduce incidence.
And in some cases they have the reverse of the effect intended - an old saying in England was “may as well be hung for a sheep as for a lamb” - as very minor crimes had the death penalty you may as well go large as you were dead anyway, if caught.
I think that's the main issue. Tougher sentencing doesn't tend to reduce incidence of a crime, or at least it's rare that it does. I appreciate that it's counterintuitive.
However, increased likelihood to be caught for a crime does reduce incidence.