Tomes are a slow monotonous guarantee. You get there as an eventuality of all your work, but the road is a tedious and boring one.

Random drops are a spontaneous and exciting uncertainty. RNG can favor you, or hinder you. You could get the best drops, or never see them.

I prefer the chaos of the latter, honestly. It's a better carrot on a stick than grinding a currency for me.

Though there's nothing that says both cannot exist at once.