It's not about how much they make NOW, it's about how much they WILL make.
SM has been setting EXO up to becoming a touring artist like TVXQ. They have yet to debut in Japan yet they are having a freaking dome tour in Japan.
It took SNSD years to only have ONE Tokyo Dome date and the same with SHINee while EXO is have 3 in a row without even having to debut in Japan.
SHINee and SNSD also only do arena tours while EXO is going into dome tours already.
EXO will definitely pass SNSD and SHINee when it comes to revenue soon now they're actually touring with big revenues now that SM is sure they can tour well. Super Junior and TVXQ are always on tour though so passing them might not happen in terms of revenue but both groups still have people going to army and that leaves EXO. SM is making sure EXO has touring power to keep them afloat while TVXQ is gone at least.