I think the game has different spell lists for every class...there can be spells on multiple lists but they are still separate lists. And the reason they can't just go add Warlock tick next to all the cantrips is because then all the catrips would be available to all warlocks when they hit that level...including non Pact of the Tome warlocks and allowing warlocks to take more than 3 cantrips they normally couldn't have, etc. It's supposed to be a strictly one time access to the cantrip lists of other classes just for Pact of the Tome. That's why it's not that simple.
Fiend and Great Old One warlocks already have differences in their spell pools, as an example: fiend warlocks get exclusive access to burning hands and command, while GOO warlocks get dissonant whispers and hideous laughter. I really don't think getting something similar for the pact of the tome warlock would prove overly complicated.
