Because they can, and they know we will buy the extra cable, along with the USB-C charger they advertised for the iPad in particular... but stupidly didn't include with the iPad. instead it was shipped with the 12watt charger and 3' cable. *Uggghhh.
For every 2 steps forward Apple takes, they take 4 steps back... or they create truly incredible products with good prices... then bomb it on prices and features on many other devices. (like the iMac Pro, new MacBook Pros, and HomePod.)
They make amazingly powerful tech, but execute many of them poorly, and somehow they screw up on adding an accessory that would truly complete/compliment their devices.