There are instances of attacks as a reaction, as in Sentinel - When a creature within 5 feet of you makes an attack against a target other than you (and that target doesn’t have this feat), you can use your reaction to make a melee weapon attack against the attacking creature.
Actually, quoting this probably would help Fenrir's answer, as when it's not the Attack action, it's specified as "melee weapon attack". But I can still see problems with that interpretation, as readying allows you to make any kind of attack (ranged weapon as well, I mean)