Because we keep on sinning.
After confession we are given a penance but it’s like what’s the point of a silly couple of Hail Marys when we are gonna go home and pray all night anyways?
Are you complaining that the typical penance is not “severe” enough? Do most Catholics “go home and pray all night?” Some do, to some extent - Third Order (secular) Religious pray the complete Divine Office daily. My wife is a Third Order Carmelite (OCDS). She prays a whole lot more than I do.
When she goes to Confession, she tells me that she identifies herself to the priest in this capacity (a priest confessing to another priest would do the same thing, as my brother, Fr. Eric Filmer (an accredited CAF Apologist), tells me). The presiding priest normally adjusts his council and penance accordingly. It’s possible (and, I suspect likely) that my wife’s council and penance is more “severe” than my own.
To whom much is given, much is expected. Would we expect a greater measure of valor from a United States Marine than a National Guardsman? We expect some measure of valor from both, but our expectations of a Marine are higher. If a Marine failed in this respect, we would hold him to a higher standard of discipline than a Guardsman.
(Full disclosure: my youngest son is a United States Marine.)
But, even if the presiding priest does not adjust his council or penance, the absolution is valid:
Is it not lawful for me to do what I wish with what is my own? Or is you envious because I am generous? [Matt 1:20] [from the parable of the workers in the field]
Our Lord preached generosity. The Church is typically (or hopefully) generous with Grace (especially under our current Pope).
What, exactly, are you objecting to?