The reason that most keyboards are tilted up at the back is that the vast majority of computer users have chairs that are too low and/or desks that are too high. Only a small fraction (e.g. those who have adjustable keyboard trays) end up placing the keyboard at an appropriate height relative to the torso.
If the keyboard is going to be up high, as it is for most users, then the forearms will be tilted upward to type, and a “positive” (up in the back) tilt is required so that wrists stay in a neutral position. If the keyboard is at a proper height (on a low desk or keyboard tray, or on a waist-high standing desk), then a negative tilt is better.
A laptop on a lap when reclining on a couch or easy chair is actually a pretty good height/slope, though the display is much too low for neck comfort. A laptop on a regular-height table is quite terrible all around.
I think the DataStealth keyboard had the right idea, though they never got off the ground w/r/t commercializing their product.