Of course you can use a cable, but they are unreliable. They tend to break, corrode, get dirty, disappear, have incomparable ends(one side USB-A, the other one micro USB, lightning - whatever) that one of the devices don't accept.
They are also bulky and the transfer speeds are slow. Airdrop can transfer gigabytes of data in minutes, which is only possible with the latest wired interfaces that only the newest devices have and not all support file transfers.
iPhones tend to last 3-5 years or even more and iPhones from different generations can transfer each other files through AirDrop. Not everyone has the money to update their phone each year and many who can update think it's wasteful to do so that they can use the latest wired file transfer method.
They are also bulky and the transfer speeds are slow. Airdrop can transfer gigabytes of data in minutes, which is only possible with the latest wired interfaces that only the newest devices have and not all support file transfers.
iPhones tend to last 3-5 years or even more and iPhones from different generations can transfer each other files through AirDrop. Not everyone has the money to update their phone each year and many who can update think it's wasteful to do so that they can use the latest wired file transfer method.