I like having the mass attached to my body, as opposed to having an awkwardly-weighted bike. Reduced unsprung weight for more maneuverability :) Much easier to hop speedbumps and the like, which is pretty useful for urban cycling. I have a light 20L pack with an internal frame and mesh for my back so I still get decent ventilation.

I like the backpack better as I can bring it with me on the last 100 ft and into my building where I shower and change, where if its pannier's I either need a backpack inside of it, or I need to carry my stuff around.

Also people steal stuff, so I don't like leaving things on my bike.

Try choosing "Office bags" from https://www.ortlieb.com/en/produkte/radtasche/

There are pannier backs with backpack straps, or shoulder straps, etc.

Although I have the more basic "back roller" standard detachable panniers. They take a second to attach/detach, nothing remains to be stolen. Best £50 I ever spent! They still look new after 2 years of daily use.