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@s.walrus
Because it would be rather poor engineering. All the weight of the bike+rider, and every bump will be trying to push the axle up, which would stretch at the chain, and cause lots of wear and loss of power. Also if the axle is knocked upwards, there is a reasonable chance it will turn and the nut can loosen. Also, as I said, as you 'adjust' the axle down and forward you very quickly start getting less contact area for the nuts.
Straight vertical drop outs will slightly tighten the chain as you drop the wheel out, whereas semi-verticals will loosen it slightly, so would be ideal for fixed gears / non-tensioned hub gears with ecentric BB or hubs. I can't see any advantage either way with deraileurs or other tensioners. If you want to be able to move the axle back and forth, horizontal movement is what you want.
(disclaimer - this is speculative and largely based on experience with nuts and bolts in general, rather with axle nuts and dropouts specifically)