Because mental capacity is finite, and knowing as much as possible doesn't work for everyone, especially if it's disconnected from actual problem solving. Reading a book cover to cover and have everything in there stick in the mind is great if it works for you, but there are plenty where it does not.
The problem isn't that mental capacity is finite, it's not (or the limit is too high to be relevant). The problem is that by spending time learning esoteric knowledge, you're only displacing time for more applicable knowledge.
