Why are antibonding orbitals higher in energy?

Due to reduced electron density between the nuclei, there is stronger repulsion in antibonding orbitals compared to bonding orbitals at the same energy level. This results in higher energy levels for antibonding orbitals. Additionally, the out-of-phase alignment of antibonding orbitals contributes to their increased energy. In simpler terms, antibonding orbitals have increased energy levels due to decreased electron density between nuclei and their out-of-phase alignment.