Because the A standard's baseline is A0 which has exactly 1m^2 area. So that nice rounding did happen, just at the baseline scale, and on area. The B series is a tad larger, B0 happens to exactly be 1,000 × 1,414 mm.
Which also makes it quite nice to know how much a sheet of paper weighs. At 80 g/m² you'll get 5 g (80 / 2⁴) for A4. Handy for quickly determining how much postage to put on an envelope based on how many pages you put inside it.
Because the A standard's baseline is A0 which has exactly 1m^2 area. So that nice rounding did happen, just at the baseline scale, and on area. The B series is a tad larger, B0 happens to exactly be 1,000 × 1,414 mm.