It's the pill that's rare both in the matrix and in real life. Morpheus had the last purple pill, yet didn't offer it to Neo. Since Neo din't question his choices or asked if there were other pill choices, Morpheus didn't say anything. If Neo would've taken both the red pill and the blue pill, he would've known that there were glitches(good and bad) in the matrix, like the hand in molten lead glitch. The purple pill would allow Neo to understand that life is full of glitches and hacks and that the system is actually trying to patch up as many glitches and hacks as they possibly can to maintain control.
The purple pill exist in real life. It used to be readily available, until it got replaced with benzodiazepines. The purple pill allows you to restructure your subconscious mind by adding any false memory or experience imaginable no matter how fictional. For example, if you want to believe you were travelling to a different universe where cartoons and smart metal robots(the ones that's metal turn soft during sex and turns tough during battle) actually exist and that a flying robot took you flying and then a flying penguin gave you superpowers, it can seem and feel like a real experience and you'll think that you actually did go to a cooler universe. However, only a purple pill can help you get awesome false experiences.
Neo: There must be another choice.
Morpheus: The purple pill.
Morpheus: But what about picking the red pill or the blue pill.
Neo: Screw it! I'm taking the purple pill!
-Years later, a movement of educated hippies who discovered the secrets of catalyzing peace and exponential learning changed the system forever all because they took the purple pill, experienced inner peace, travelled to different universes, and stuck together to change the system to allow people to become as smart and happy as they want while being individuals. Even if things sort of resemble a rerun of the sixties.-
Neo: I knew the purple pill was worth it. Purple pills for all!