My assumption was that blindsight and tremorsense served as a substitute for sight. For example, spells the require you to see the target. A creature with, for example, echolocation has good enough hearing that it can completely replace vision. Creatures without these senses still have normal hearing, it's just that a creature with blindsight can perceive as good as if they could see the thing in their blindsight range. You can detect someone via hearing, but you need to actually see them to get the details on who/what they are.

Blindsight itself is a bit frustrating, since it doesn't describe how it works or what it can do. A creature with blindsight can read, for example, because the assumption is that it's a type of "sight" and nothing says they can't. But there would certainly be cases where it wouldn't make sense for them to read via blindsight, such as those using echolocation.