The certification authorities assume that the chance of an engine failure at the critical point (V1) is a sufficiently remote enough possiblity to allow a reduced screen height and subsequently not too much of a risk for you to fly the N-1 profile. You may also see that reverse thrust is may be allowed on a wet runway as well. The reason for this is to boost your wet runway performance so that you can carry a very similar (or notionally greater, but reduced to Dry) load to that if it was dry. In practice it means that you don't have to chuck people off just before take-off because it's just started to rain.