Because silkweed often catches on the barb itself😉
To be honest I tend to use a low pop-up over silkweed, just high enough to keep the hook away from the strands and detritus.
You want a pop-up that will lift the hook upright, and you don't want to use too much weight as a counterbalance or it will get dragged down again into it. I hate to say it, but this is exactly the right place for a 'chod' or silt rig, or a standard pendant set up with a coated or uncoated braid hooklink.
In winter carp can stay holed up in a particular area. It may be near dead weed, snags, natural food, drop-off features like gravel bars or plateau, and is NOT always the deepest water.