They don't prevent walking any more than living in the suburbs do. But there's a slight barrier waiting for an elevator, or walking down stairs, etc. There's a reason that grocery delivery services sprang up in places like NYC first and most prominently instead of, say, Pittsburgh.
While they enable density, certainly better than the suburbs do, they're not a great solution. For large cities you want to look at a city like Paris (Ignoring the central business district which is set aside like an upset child that needs to think about what they've done) for even better and more appropriate density.
I think often times people assume more skyscrapers == more density therefore good. But I think that level of density isn't healthy either, it's just more healthy in comparison to the abysmal and wasteful suburbs.
Also your statement is a bit contradictory. If the skyscraper apartments are an extremely low portion of units, how do they enable density if there are so few?