If you may allow me to explain. Cell, Dr. Gero designed you to be the Perfect Lifeform at that given time, but he forgot to factor in one thing: the potential for others to grow beyond their current states. So while you were temporarily “perfect”, Gohan’s potential being unleashed marked the end of that period, which is similar to the Granolah paradox in DBS, where Granolah wished to be the strongest in the universe, but he was only the strongest in the universe at that specific point in time, as he was quickly surpasses by Gas and Black Frieza.
