When people are made to do good things due to irrational reasons, they may be made to do bad things due to the same or similar reasons the next day. A religion can teach to love and it can teach to hate. Why not discard such a tool and let people make their own decisions with better reasoning other than fear/love of God or wanting to have a better afterlife?
Second, the essential data is rarely objective or complete. There are very few datasets where a subjective value judgment on some information can be avoided; if you incorrectly devalue some data, your "rational decision" can turn out to be very problematic indeed. So not only are most humans incapable of performing basic rational deduction, they also often lack the perspective necessary to adequately value a subjective dataset, where "adequate" means interpreting it to be compatible with general social cohesion and happiness.
The tried and true traditions of the previous generations of a successful society may err in some smaller things, but in most things, they will be reliable. Young adults (< 50) often lack the maturity and perspective to properly understand the decisions they're making. They'd do well to listen to their elders and try to learn from them.
All of this culminates in "religion" or something very close to it, and it's essential to social stability. If you don't provide one, a replacement will automatically generate. People will find and adopt a belief system as absolutes. You can see this in the "secularist" society of today, that adopts what they perceive to be "scientific consensus" as effective religious tenets, or in the "social equality" segment that adopts their interpretation of "diversity and equality" as effective religious tenets.