One response against this is that making tomes stack will trivialize the levelling system causing more people to disregard events like the lower level CMs and making the F2P base interact less with the paid base and believe me there are a lot in that he latter.
The Exp tomes were made to speed up the grind but speeding it up too much will disincentivise play. One of the frequent problems cited by high-tier players was that once you reach the top there’s nothing much to do although IMC is supposedly working on a solution for this.
I can see the point of creating level boosters and even Max Level Boosters but often I see this in games which have a dearth of content locked behind a paywall and a different system for handling “endgame” levelling. (I mean, I played GW2) There are no such systems in this game.
I can imagine that this would be unsatisfactory for you but I hope the perspective helps.
Edit: 'cuz I’m an idiot