The reason it is thick is because it supports 65W charging. Apple did the same with the USB-C cables that shipped with the pre-MagSafe MacBooks. It was a thicker cable that supported 100w charging but was only USB 2.0.
Or, why Apple manages the same in half the footprint?
Or, why someone would expect that a cable that came with a pair of headphones actually charges things at over 65w?