This has been discussed a thousand times. It's because they do not charge transaction fee, period.
exactly, you can basically sell your own btc to yourself with no financial impact and inflate "volume" as much as you want.
common people in china left bitcoin in december selloff after it was banned from business use.
the only ones left are whale trade speculators who use high frequency trading bots on 0% fee exchanges to inflate volume and exert influence on other exchanges by pretending to be a "volume leader" and then do exploitative/manipulative trading on those exchanges.