The world club cup is fifa's club cash cow, the only thing they have after world international tournaments, Klopp comes across that it's a privilege to be invited, and that they are representing the premier league, England, the UK and Europe, and they couldn't say no. In other words, they could have, but it wouldn't have gone down well.
The bigger picture is, this competition was planned to include the winners of the champions league (who ever it was) before the champions league final was played. Moaning about it now is all a bit too late, they knew they had premier league games, they knew they would be in the Champions league, they knew which cups they were in, they knew if they stayed in the EFL cup to the quarter finals it would be a clash with other games, why not bring it up sooner and ask that a provision is made.
They have 35 players registed with the premier league, plus other EDS and under 21 players. They should just bloody well get on with it, but because they are the media darlings, it will be poor successful Liverpool, they must be looked after.
We know that wouldn't have been the case if it was us.