Except when I was looking at the Magic section and the casting time:
Casting Time
Most spells require the
Magic action to cast, but some spells require a Bonus Action, a Reaction, or 1 minute or more. A spell’s Casting Time entry specifies which of those is required.
You'll note ready is not listed. This leads me to ready just being an extension of using the magic action, but not explicitly stated as so. Is it a concrete iron clad reading? No, but it makes more sense, to me, than to allow ready to break a rule set for the magic action without explicitly saying it can.