Many reasons. To the point of darkness because it's a thin line between a light buzz and over-the-top-cannot-even-say-my-name
> Why is this so common?
human nature. Not only human, actually, monkeys drunk on fermented fruit, etc [1]
> Has this always been the case?
Yes
> Will this change?
No.
[1] http://www.australiangeographic.com.au/topics/wildlife/2011/...
[1] http://www.dailymail.co.uk/news/article-2229334/Fifty-drunke...
Most people are already really bad judges of how much they can drink before reaching various stages of intoxication while they are sober; it only gets worse when they're drinking. This is further complicated when mixing drinks with different alcohol contents.
So while sober you might guesstimate that you'll be "pleasantly buzzed" after about three drinks. Then you have one or two drinks and your self-reflection skills get worse and you become convinced four or five is fine too and six is really only just a bit more than three, so that'll be fine too.
It's generally an aspect of maturity to learn not to trust "drunk you" when asking yourself how much you should drink but humans are humans, so even adults screw up a lot.