Because Each beer is one standard drink for an average person. 2 standard drinks will cause you still to be under the Blood alchol limit. So they say 2 beers just so they're saying, "i did drink but i oly had 2" thus really trying to say "i shouldn't be over"
There are ways in keeping under the blood alcohol limit. Like if your limit is .05% then that is equivelent to 2 standard drinks but the way it works is:
You can have 2 standard drinks in the first hour
and then one standard drink every hour after that and you should still be under the limit.
But again body size, body mass, sex, blood pressure, what you ate before, how fast you drink and etc are also factors that should be in consideration. These are just for an average person.
Why has this become the common sense for drunk people?
They're psuedo lying... saying they drank but they're not drunk. And it's common sense that 2 drinks shouldn't allow you to be over the limit since generally speaking people are still quite coherent after 2 beers.