Because it is; I frequently use that service down towards Gatwick.
After Hitchin the display boards are showing all carriages red/orange as full, but south of the river every carriage is showing green with seats.
From Hitchin you can't move through the train, from London Bridge the rear third of the train is empty.
You seem to be overlooking that for most of GTR's customers those trains go to the wrong station.
Those trains go to Kings Cross but the vast majority of the customers on that train don't get off at St Pancras but continue on to one of the Thameslink stations. The train is still full and standing when it leaves St Pancras
The loosers were all of GTR's customers who ended up standing on freezing cold platforms and then had to catch an overcrowded service which didn't take them where they wanted to go.
It rather seems like you should stop making assumptions.
And here is another example of GTR annoying their customers by playing at train sets -
http://www.realtimetrains.co.uk/train/W64332/2018/10/29
What did the failure to stop at St Neots save? Two minutes. Meanwhile all the delayed customers get kicked off onto a freezing platform and delayed another 15 minutes.