Did you ever take a look into an x-ball pit? How many people do you see there on average? Would say something like 10...
Someone needs to be there to fill pods, clean players off, repair broken guns and what not... Why do you have 10 people on a 7man team in the first place? Probably, because sometimes someone gets injured, has other commitments to fullfil, can not afford to go to the event and so on, and it's just the same in CPL/SPL.
An other thing: Someone stated Div1 teams usually do not play all the events. If Div1 is turned into something like CPL/SPL, all the teams will have to play all the events or otherwise loose their spot...