Theologically- God, via the Church, says so. Easy.
Secularly- disrupts society; society, when disrupted, impugns the progression towards perfection of culture. Subsidiarity (you can argue this from the U.S. Constitution) demands that individuals bear responsibility. Anytime the individual, who has responsibility for self first and foremost, necessitates a previously needless interruption by the higher spheres, it disrupts society.
In The Kite Runner, the father tells the son that the worst sin is theft. Or maybe he said all sin is theft… and it’s true in many ways.
Adam stole from his descendants the chance to be in the Garden, communing with God, by evil.
What is theft, but a physical representation and affecting of a lie? “This is mine”, when it’s most certainly not. So theft is lying in a degree.
This is why St. Thomas Aquinas argued that if they were to put to death forgers, they should put to death heretics, since they forged false doctrine.
Just don’t tell PBS.
