The short answer

Skyscrapers are made of glass because glass is relatively lightweight, looks modern, provides a lot of natural light, and is easy to maintain.

The long answer

Two important innovations allowed for these giant glass skyscrapers to become so common: mass-produced steel and air conditioning.

Before the Industrial Revolution, making steel was an expensive and time-consuming process, so the material was reserved for small-scale productions like knives and razors. But when new methods were invented to reduce the fuel costs and effort involved, steel became a popular building material. Since steel beams could handle much heavier loads within the internal frames of buildings, the exterior material did not need to be so structurally strong. Thus, skyscraper glass!

But a giant, glass-wrapped building presents a problem: heat. Glass allows light to pour through and raises the temperature inside of the building. Early glass-wrapped buildings struggled with this challenge. In the summer of 1851, the "Crystal Palace" was constructed in London to house the Great Exhibition. Despite efforts made to cool the building, "overheating became a major issue ... and was the subject of frequent commentaries in the daily newspapers," as noted by Henrik Schoenefeldt.

The rise of commercial air conditioning in the early 20th century solved this problem and allowed for these giant glass towers to pop up in greater numbers with more comfortable occupants.

Now the next time you're walking down a city block with your neck craned to look at the glass 'scrapers (I'm coining this nickname), think of steel and A/C.

But...

Perhaps glass skyscrapers will fade in popularity. There are serious downsides to all-glass exteriors. With all that heat in need of cooling, a lot of energy is used to cool these buildings. Additionally the glare caused by light reflecting off of the windows can dramatically increase the heat in the surrounding areas. One building has allegedly caused parked cars to melt due to its reflected rays, earning the nickname "fryscraper."

Sources

BBC. (2014, May 27). Could the era of glass skyscrapers be over? BBC News. Retrieved October 19, 2022, from https://www.bbc.com/news/magazine-27501938

Finnigan, S., Charnish, B., & Chmielowski, R. (2015). Steel and the Skyscraper City: A Study on the Influence of Steel on the Design of Tall Buildings. The Future of Tall: A Selection of Written Works on Current Skyscraper Innovations. Retrieved October 19, 2022, from https://global.ctbuh.org/resources/papers/download/2409-steel-and-the-skyscraper-city-a-study-on-the-influence-of-steel-on-the-design-of-tall-buildings.pdf

Schoenefeldt, H. (2019, May 17). How glass skyscrapers took over the world–and why we need to stop them. Fast Company. Retrieved October 20, 2022, from https://www.fastcompany.com/90350542/how-glass-skyscrapers-took-over-the-world-and-why-we-need-to-stop-building-them?position=4&campaign_date=06072020

Skeleton frame. Designing Buildings. (2020, October 30). Retrieved October 19, 2022, from https://www.designingbuildings.co.uk/wiki/Skeleton_frame

Valle, G. (2021, November 20). Why do skyscrapers have so many windows? BuilderSpace. Retrieved October 19, 2022, from https://www.builderspace.com/why-do-skyscrapers-have-so-many-windows