Because we do directly experience our own awareness. There's nothing intervening.
I suggest that you, as an adult, have forgotten just how much learning it took for you to be able to speak coherently about your own private behavior. These things, which you claim are the only ones we directly experience, are far more difficult for us to describe than are objects in our environment. It's easy for us to point to circles, red, fuzzy, distant or heavy; describing pain is difficult ("is it a stabbing pain, a throbbing pain, a dull pain...?" note that we use terms that are based on things in our environment to describe the pain inside us!). Describing love, more so ("when you say you love me, do you mean the same thing I mean when I say I love you?").
There is quite a bit intervening when we experience our own awareness, but you have forgotten it, since most took place while you were still becoming verbal. To learn "dog", all you had to do was to be able to learn to agree with your verbal community, that this group of objects were dogs, and other things were non-dogs. There were any number of dogs around to point to, and virtually all of your verbal community could see them, just as you could. But... when learning your awareness, your pain, your love, learning any of the feelings, between which and you there is "nothing intervening"... the only way to learn those was from people who had no access to your feelings, and to whose feelings you had no access. So you learned them through the public behaviors, objects, and situations that accompany them. If our feelings are the same, it is because similar bodies (including nervous systems, for you reductionists out there), in similar situations, can be assumed to behave similarly (assumed, not proven). All the similarity is in the physical systems; to the extent that one assumes a "consciousness" that arises, separately from the behavioral response to an environmental stimulus, there is no reason to assume that this "consciousness" (in scare quotes to distinguish it from my behavioral definition, which is actually coherent) is the same from person to person. If it is something that is not explainable by the self-referential feedback loops, if it is a mystery, then we have no reason to assume that we are talking about the same "experience" from person to person. The reasons we have to assume things are similar are all included in the feedback loop/behavioral version of awareness.
Long story short... (too late!), we actually have considerably less confidence in our introspective accounts than in publicly available (I won't call them "objective", since the objective/subjective distinction is a superfluous can of worms) relationships.
