Skyscraper are not a good or a primary target. Why bother with a tower if the same number of people can be hit easier on a surface - a demonstration, Sunday market, transportation hub etc. Also I suspect that a lot of conventional explosives is needed to topple one, and it also requires to have a specific faults. You need at least half a ton of explosives to maybe do something (basing on the amounts russia uses in their ballistic missiles against cities), and that is not guaranteed.