A skyscraper is a large building packed with many floors, usually designed for office, commercial, or residential use.
Skyscrapers usually only need to have the first few floors secured and under watch, still leaving more than enough room in upper floors. There will be no shortage of living space, even with the equivalent of an entire small town population living inside. The roof would be a place to grow food.
Because of their massive size, skyscrapers are extremely difficult to completely clear out if the undead are already inside, even with a large team. Skyscrapers generally do not supply their own power, so doing this would be a hassle.
Just because there is a lot of room in a skyscraper does not mean all of it would be useful in a post-apocalypse. Most skyscrapers are made for business uses, so there will be few resources that people are able to use inside. These buildings would not be good bases since they would be located in cities with lots of infected surrounding it, and it would not be able to support the large number of potential survivors that could fit in it. With little usable resources inside, people would need to venture out to scavenge supplies. Even if ground entry ways are barricaded and secured from zombies, that also severely limits ways for people to leave and enter. And if the undead do get in and try to stumble up the stairs, which they will never stop trying, those inside would be trapped on the upper floors with no way down (even elevator shafts would lead to floor packed with them).