There'll be dozens of different reasons -
People want to sit with mates for a change
People want a better seat for same price
People went straight to the map and didn't realise thier seat was locked in for them already
People with ST for next season
People who tried to activate thier card and when it wouldn't work, bought another seat
Probably loads more reasons . You'll get people on here saying they are all stupid and none of this is the clubs fault etc... However in the normal private sector if you failed so miserably at predicting what your customers would want, failed so miserably at telling them how they can easily get what they want, failed miserably at allowing them to get what they want, and repeated these failures every single time you had to deal with your customers, whilst simultaneously making thier lives unnecessarily confusing and inconvenienced ...someone would lose thier job over it
Yet at SAFC the club clearly don't give a fuck about any of this, and most fans will blame other fans as a default position. Anyone pointing out these failures is branded a wailing fanny
Hence why the club get away with
1000s of fans literally standing out in the rain all day to get something they could, and should, be able to get online in a matter of minutes. Nothing will change