Because computers don't actually do arithmetic, they simulate it. When you take 2 sticks, add 2 sticks, and obtain 4 sticks, that's arithmetic. Having a raised flag with one lowered flag to the right and the left of it and then changing this configuration to having a single raised flag with two lowered flags right to it, and interpreting this charade as having added 2 to 2 and obtained 4 — that's just a simulation. It didn't actually have added 2 and 2 of anything.
Well, if I write "2 + 2 = 4" on a piece of paper, am I doing arithmetic, or merely simulating it? After all, there are no sticks involved.
But I'm also willing to accept that a computer is simulating arithmetic, and not actually doing it. That makes no difference to me, as long as the output is the same. In fact it's kind of a boring distinction to make.