It's just an idealized problem for a Physics I course. Airdrag, density, changing gravity with height, curvature of the Earth and all those other variables you can think of isn't necessary for a problem where the distance traveled is only 55m. The error only becomes apparent at much greater distances, heights and velocities.
You can spend all day accounting for the different variables, and it'll probably give you an answer within 1-2% of the one you get with the kinematics formula.
It's never 100% correct anyway, most things end up being approximations.