Skyscrapers are far more simple than a tunnel/most infrastructure projects. A skyscraper is some pipes in the ground and a big stick of concrete up into the air with some steel beams coming off the side to hold the floor up.

A tunnel is the logistics of tons and tons of skyscrapers next to each other (not counting station structures) that need to avoid utilities installed in the 1800s, avoid causing settlement of nearby buildings while making a hole under them, and can’t cause any disturbance to the roads above. Going from basically nothing to a year or two of alternative analysis, two years of design, and four years of construction is hasty.

Keep in mind the economies of scale thing, we don’t have a big metro design industry, we need to mobilize/purchase all sorts of specialized equipment, etc, all for a short project.