I think our short attention spans probably come from the fact that many more things in the world demand our attention, thus requiring us to divide our limited attention span pool up amongst many things.
That doesn't necessarily explain teenage or childhood short attention spans, though, except that maybe since they're still absorbing and trying to process a lot of information, they feel demands that adults wouldn't since adults have a steadier understanding of the world and their bodies, since their subjectivity doesn't necessarily change as much.
The question I always like to pose: do we really have enough information to actually claim our attention spans really have gotten shorter or is it just the usual putting forward the idea that everyone lived/existed as some kind of ideal norm, and we've deviated from that norm because this, that and the other thing. . .you know, like there was a golden age of attention span, which we've drifted away from because modern technology has corrupted humanity?
BTW, sometimes I find that Google doesn't work as efficiently as I like. Sometimes I need to go to Wikipedia, Amazon and even Google Books (which I guess is still officially Google) to find the types of information that I'm looking for.
Then again, I probably look for more obscure information than other people. At the same time, many of the commercial/sales Websites for not as obscure stuff get in the way of me efficiently getting as minimal biased information as I can.