According to Michael Arrington touchscreen Tablet isn’t something we have only seen in 21st century, but also in 1990’s. Samir Arora (Picture Below) is holding Pen Mac which was touch-screen enabled, and it is also project that was never fully completed.
Remember those old times when PDA’s with pens were like Nintendo 1.0? I was 10 years old 
So if this was invented by Apple back in 1990’s what is holding in next 20 years, only Apple knows.
Credit:TechCrunch by Michael Arrington