I think that's because the cat is supposed to be burning the residual hydrocarbons and carbon monoxide. This requires heat from the exhaust gases to happen, and, (guessing here), produces more heat in the process. The exhaust needs to get very hot, this is not going to happen other than on a longish run.Set VCDS to monitor bank 034 (cat temp) which showed 340 degrees at idle and then measured cat temp with an IR thermometer.
Front end of cat 112/113 degrees and the back was 118/122.
Used an assistant to bring revs up to a constant 2,500rpm which increased VCDS cat temp to 505 degrees.
Front end of cat now reading 122/124 and back showed 124/128.
Most videos imply that the temperature at the rear of the cat should be considerably higher if the cat is working correctly.