Because - for there to be a vertical line - time would have to stand still !
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If a graph shows distance on the vertical axis and time on the horizontal axis, and the speed is steadily increasing, the line representing speed will be a straight line.
a vertical one
constant speed
The straight horizontal line on the graph says: "Whatever time you look at, the speed is always the same". This is the graph of an object moving with constant speed.
Constant speed ... zero acceleration.