Rather, in order to hit further away targets you need to initially fire at more than 45 degree. A higher trajectory gets you into thinner sections of the atmosphere more quickly so you lose horizontal velocity to air resistance less quickly. Air resistance doesn't seem too big a problem to us in our day to day lives, but increases as the square of velocity so at the speeds that artillery shells travel it will tend to start out as a much stronger force than gravity.

What ICBMs (and space_bound rockets) do is to travel vertically initially to get out of the thickest part of the atmosphere before starting to put on rotational velocity. You'll notice the article you linked specifies a reentry angle of 22 degrees, not a launch angle of that. A missile has an advantage over artillery in that it can apply thrust over time, and wait to start accumulating horizontal velocity until it has gained some height.

EDIT: A better way to explain things. When a bullet leaves the barrel of an artillery piece the forces on it are dominated by air resistance. As it travels it's kinetic energy decays exponentially with distance due to air resistance, and eventually it's traveling slowly enough that gravity becomes an equal source of acceleration as it settles into terminal velocity.