It is so because we are interested in the substrate (the thing that is undergoing the reaction). since, the concentration of substrate is (as in most cases) very very high. it only depends on enzyme, how fast it can convert (popularly known as, turn over number).
alternate-analogy_look at this reaction more as a surface reaction. all enzyme does is provide an active site (surface) since surface is a limiting factor here. the rate in independent of the concentration of substrate. however, at very low pressure in case of surfaces, rate of reaction also changes.
thus enzymatic reactions are first order when substrate concentration is much much higher than that of enzyme.