Can the Touch Attack from Calcific Touch Be Used As Part of a Full Attack?

This came up in play recently and I want to make sure I understand the rules here so I don’t make the same misinterpretation for other spells.

I focus on full attacking in combat with six attacks, and picked up Calcific Touch with the intention of being able to use the once-per-round touch attack you get from the spell as part of my full attacks. But when I tried to do this, it was pointed out that the touch attack granted by the spell is a special action that cannot be used as part of a full attack; it cannot be paired with any other attacks.

I’ve gone through the rules on touch spells and holding charges and such and think I am still missing something. Could somebody explain in a bit more detail why that wouldn’t be allowed?