Probably because many guitar players refer to the jack socket on the guitar as "input".
Most of the musicians I know call a Female Jack an input, because they will insert there a cable, so for them insertion is "input". Wrongly, they don't call it by the audio signal flow.
Most people also call the Headphone Amplifier jack output an input, same reason , because they input (insert) there a TRS Jack.
Also people call a TRS Jack a "Stereo Jack", when it's just a connector with 3 contacts, it can be used for mono or stereo signals, or any other duty that needs 3 contacts.
An audio signal can be mono or stereo, balanced or unbalance, but a connector is never stereo/mono balanced or not, it's just a connector with certain number of contacts.
The same for cables, musicians call it a "Mono" cable or a "Stereo cable"....
Well I have this problem every year in my classes, all the students do this mistakes, and they have an hard time to chance for the correct terms and to actually follow the signal flow of an audio signal.
But I do my best to push them into understanding the correct terms