A canister filter is a closed system. The water is FORCED through the media. This means that every bit of water goes through the media. In HOBs and UGFs the water will go over or around the media, or in UGFs one spot will become clogged with debris and the water then rushes through another area, reducing how effectively the water is cleaned. This is mainly an issue with mechanical filtration, especially fine mechanical filtration and polishing.
I've owned canisters with media bypass, and I've owned HOBs and UG (OE-RFUG specifically) with zero bypass. You are making generalizations that are not even close to being universal. Not everyone with a canister is using top-of-the-line Eheim canisters with zero bypass, and likewise not everyone with UG is using a crappy air-driven undergravel and vacuuming once every few months.