Adding to PBJ's comment:
1) Not everyone is the same Height.
A young kid, would have a real hard time seeing the monitor... if it was at the height of an adult man... even with a booster stool.
2) Saved space
Large CRT monitors took up a lot more space in a cabinet. Angling them reduced the depth... and probably helped with heat dissipation too.
3) Isolation
In older cabinets, like Ms Pacman... you stand in such a way that the cabinet sides are almost surrounding you. It not only helped
to amplify and keep the sounds local to your ears... (and helping to keep other sounds out) But it created sort of a "Blinder" effect...
where you were not distracted by the games / people...etc.. next to you. It helped with focus, and escaping the world around you
more completely. More "Immersed", in the game / action.
You will notice, that Showcase Pedestal machines, used vertical monitors. Size of the screen + far greater distance away from the display,
changed the height viewing issues. However, you also lost that Immersion in the process. You dont realize how much you miss it... until
its gone...