Inconvenience or because we can or of the risk we won't be able to bail out of a cruiser in an ambush or to immediately pursue or otherwise engage a suspect. The "reasoning" is nonsense, but it still persists despite efforts to discourage it.
The Maryland Police Training Commission conducted a study on the time it wook to bail out of a cruiser with or without a seatbelt. The dreaded fastened seatbelt took much less than a second to unfasten in the moment of truth.
Whil Calibre Press has a variety of horrifying videos and photos on hand, probably the worst I've seen was a prisoner transport van involved in a low speed collision with a fixed object. Prisoners were fine and minimal, if any, l damage to the vehicle, but the deputy wasn't wearing a seatbelt and became a name on the wall the next year.