I don't really understand why some philosophers claim that it's impossible to be absolutely certain of anything except my own consciousness. Isn't that absurd? Personally I believe that this kind of extreme skepticism is just wrong and that it's possible to obtain 100% certainty. I believe that we're just looking at what we don't know in the wrong way. The best argument I have heard so far is: we can't be 100% certain of anything because we don't know everything. I mean, I don't know why I perceive colors, does that mean I can't be 100% certain I perceive them. What do you think?
In our philosophy, the sentence, We can't be 100% certain of anything because we don't know everything, is not completely true and needs revision:
We can be 100% certain of the existence of a thing but we cannot be sure of its properties because we don't know everything about that thing.
For example, assume we close our eyes and somebody puts a thing in our hands. We would try to touch it, smell it ... to imagine it with our experience about things. OK! Then we are sure there is a thing in our hands, but what is it? What are its properties? There are maybe many other questions we need answered about it.
There is a very nice poem from molavi in the book, masnavi:
The elephant was in a dark house: some Hindús had brought it for exhibition. In order to see it, many people were going, every one, into that darkness. As seeing it with the eye was impossible, (each one) was feeling it in the dark with the palm of his hand. The hand of one fell on its trunk: he said, “This creature is like a water-pipe.” The hand of another touched its ear: to him it appeared to be like a fan. Since another handled its leg, he said, “I found the elephant's shape to be like a pillar.” Another laid his hand on its back: he said, “Truly, this elephant was like a throne.” Similarly, whenever any one heard (a description of the elephant), he understood (it only in respect of) the part that he had touched. On account of the (diverse) place (object) of view, their statements differed: one man entitled it “dál,” another “alif.” If there had been a candle in each one's hand, the difference would have gone out of their words. The eye of sense-perception is only like the palm of the hand: the palm hath not power to reach the whole of him (the elephant). The eye of the Sea is one thing, and the foam another: leave the foam and look with the eye of the Sea. Day and night (there is) the movement of foam-flecks from the Sea: thou beholdest the foam, but not the Sea. Marvellous! We are dashing against each other, like boats: our eyes are darkened, though we are in the clear water. O thou that hast gone to sleep in the body's boat, thou hast seen the water, (but) look on the Water of the water. The water hath a Water that is driving it; the spirit hath a Spirit that is calling it.
I hope it was useful.
If by 'absurd' you mean 'weird' or 'unexpected', then sure, yes, this is a pretty weird claim: until we encounter these kinds of arguments in a philosophy class or book, we are all convinced that there is a reality out there, just as we perceive it. And in real life, we do make a difference between things we know for certain, and things we merely believe to be true. So, yes, in that sense the claim that nothing is certain (except, for one;s own consciousness) is a pretty 'absurd' claim.
Also, before you edited your question, you called the claim 'depressing'. And sure, yes, the claim is also 'depressing' in the sense that as human beings we like certainty, and this claim says that almost nothing is certain.
OK ... but so what? Especially given your claim that "I really don't understand why philosophers would say that ..." I get the feeling that you're trying to argue as follows: "The claim that we may not be certain of anything existing except one's own consciousness is absurd and depressing. Therefore, the claim is false"
But this is surely a fallacy! I mean, if true, climate change is depressing .. but that does not mean it's not true. Lots of absurd things happen in quantum mechanics, but that does not make it false.
In short, things are absurd, depressing, and true, possibly including the claim you're focusing on.
Finally, I completely agree that
we can't be 100% certain of anything because we don't know everything.
is a really bad argument. That's like saying: "Not everything is an apple. Therefore nothing is an apple." It's a real shame that you were not told any better argument. In fact, I don't know any serious philosopher who makes that argument for the claim that nothing (except our own csns) is certain. The classic argument comes from Descartes, and that's a much better argument. Do you know it?
One classic argument is that I cannot be absolutely sure that I am not a brain in a vat, hooked up with wires to a computer sending signals into my brain. In this scenario, the computer provides me with a simulated environment that I think is reality but it really is not (think The Matrix). Unlike in The Matrix, though, the simulated environment could in principle be completely unlike reality; for example, maybe there is no such thing as gravity in reality. Hence, for almost anything, we cannot be absolutely sure it exists in reality. (However, it should be pointed out that David Chalmers has argued here that such a virtual reality would still be truly "real" in important ways.)