Maxperson
Morkus from Orkus
It's not an exception. It's an explanation for how spells can work the same way as other readied actions. If you tried to cast a whole spell in response to a trigger, the triggering event would be long over by the time the spell went off. "Holding the energy" is just equivalent to holding an arrow drawn.
Spells are called out differently than other actions. Were they all supposed to be the same, spells would not have been mentioned separately at all. There would be no reason to call spells out. Spells take no longer than moving or swinging a sword. The way it is written shows spells to be an exception.