Some exchanger list hundreds different kind of coin and token because that exchanger doesnt care about the project. Some exchanger just care, developers team can afford paying a listing fee.
exchanger didnt developt coins.its become responsibility of developers team.but usually exchangers has policy they will delist coins that didnt developted well by their team.
Agree,exchanger also is very strict with there policy they will always monitoring every coin movement if they will see the Coins has no good volume they will inform the trader's that they will delist the tokens.