Why is deorbiting impractical for a final stage reaching Geosynchronous Transfer Orbit (GTO) or GEO?

Due to the immense fuel penalty required to slow down from such a high velocity.

Geostationary Orbit (GEO) and the associated Geosynchronous Transfer Orbit (GTO) are significantly higher in altitude and require much greater orbital energy than Low-Earth Orbit (LEO). The velocity required to maintain these high orbits is substantial. To execute a controlled deorbit burn, the stage would need to expend a massive amount of propellant to intentionally reduce this high orbital velocity—a penalty so significant it would severely compromise the mission objectives or require a much larger, more expensive rocket to begin with. Therefore, rather than wasting fuel for an impractical deorbit, the standard procedure is to utilize the remaining propellant to propel the hardware into a graveyard orbit, a higher, safer, stable disposal orbit.