Why are so many ramps on dropbars tilted downwards?

This is a function not just of the shape of the bars and brifters, but of how the bars are clamped into the stem, and how the brifters are clamped onto the bars, as well as various other factors to do with fit and grip. There's a lot that can be adjusted without changing the bars.

Modern bars seem to have a tighter upper bend than many older bars, and often have a straight(ish) section in the drops. The tighter upper bend makes for a more obvious flat region of the ramps, and setting this horizontally (by loosening the stem bolts and rotating the bars) is a good starting point which will minimise the region in which your hands can slip downwards - of course they could still slip forwards. I've seen it angled significantly up or down. Very slightly up might help though it wouldn't suit me. Of course this changes the effective stack, at least when you're on the hoods.

My combination of Sora brifters on 2017 Genesis stock bars has no downward bit between ramp and brifter that I can measure wherever I put a spirit level. In fact because of the shape of the hoods, to get a flat top the ramps are tilted very slightly down. On these bars the upper bend is more than 90°and quite sharp. For reference the clip-on aero bars are angled up about 6°

By clamping the brifters high on the drops you can further reduce the dip before the hoods, but you need to be careful here or the brake levers may become hard to reach from the drops. You may have a reach adjustment to counteract this.

The specific issue with your hand sliding forwards/downwards may be improved by a number of things. Perhaps your bartape is slippery, at least in combination with whatever gloves (if any) you use. Suede-effect gloves are better than bare skin in the wet, so long as they fit well. I've got my bars double-wrapped above the hoods with quite firm tape, almost rubbery on the surface, over the top of softer foam tape. This may help but it may not.

How you grip the bars makes a difference too. I have big flat hands so may have more options than you but here are some ideas: When riding on the hoods I generally have 1-3 fingers behind the hook (top row). I can still cover the brakes with 2 fingers, though to have my ring finger in front of the hook (top right) requires the brakes to be well-adjusted. It's easy to move into a 3--4-finger braking position (bottom left) without even letting go of the brakes. This is good for steep wet descents, maximum braking if the pads are getting worn, or just avoiding tired hands/forearms after hours of riding; this is a tourer and often laden. What I rarely do is the bottom right. It's quite a secure position, except that gripping too tight tends to squeeze my hands forwards especially in the wet. I do often hold the bars this way when riding one-handed, e.g. eating with the other hand.