They probably weren't built in a reasonable amount of time. St Paul's cathedral took 35 years and "was the first English cathedral to be completed within its architect's lifetime." [1]

[1] http://www.engineering-timelines.com/scripts/engineeringItem...

I try to remember these things when I look at the modern cathedrals, in Germany's case the new Berlin Airport, the new Stuttgart railway station, and, recently completed, the Hamburg philharmonia building - also called the German Bermuda triangle for its ever increasing costs and time-frame.

In the long term, this was the normal way to build large structures, and hopefully future generations will look at them and still be able to enjoy, even if we have to pay more and more and keep waiting.

The Hoover Dam is a similar amount of material to the Great Pyramid. Took 5 years to build.

Three Gorges Dam is about 10 times the material.

Then there was the London to Birmingham railway line, built largely by hand, which was supposed to be more work by less people in less time.

"Peter Lecount, an assistant engineer of the London Birmingham railway, produced a number of - possibly hyperbolic - comparisons in an effort to demonstrate that the London and Birmingham Railway was "the greatest public work ever executed either in ancient or modern times".[4] In particular, he suggested that the effort to build the Great Pyramid of Giza amounted to the lifting of 15,733,000,000 cubic feet of stone by 1 foot (say, 450,000,000 m3 by 0.3 m).

The railway, excluding a long string of tasks – drainage, ballasting, and so on – involved the lifting of 25,000,000,000 cubic feet (say, 700,000,000 m3) of material reduced to the weight of stone used in the pyramid. The pyramid involved, he says, the effort of 300,000 men (according to Diodorus Siculus) or 100,000 (according to Herodotus) for twenty years. The railway involved 20,000 men for five years. In passing, he also noted that the cost of the railway in penny pieces, was enough to more than form a belt of pennies around the equator; and the amount of material moved would be enough to build a wall 1 foot (305 mm) high by one foot wide, more than three times around the equator."

https://en.wikipedia.org/wiki/London_and_Birmingham_Railway#...

The Hoover Dam also finished under-budget and ahead of schedule.

Building pyramids out of single stones is still very labor intensive. So modern technology might save a lot of workers in transporting the stone, but they still have to be placed individually. To see what modern technology can achieve, look at modern construction sites, like skyscrapers or especially the work done at the spillway of the Oroville dam after it broke. Large parts of the spillway had to be replaced in a couple of months, laying enough concrete to build a pyramid - I have to look up the exact numbers to see how large exactly. Additionally a huge amound of debris was removed from the waterway. All this by a few hundred workers working in 3 shifts. Check out the construction videos here: https://www.youtube.com/user/calwater/videos

For one thing, we don't operate at similar scales of human engineering. The Romans, for example, had huge armies. But the entire army was also experienced at engineering and building, to the point where whenever they weren't fighting, they were building fortifications and bridges, which gave them terrifying speed and technological superiority.

Build an army of construction workers, engineers and craftsmen today and you can build some pyramids. Hell, skyscrapers go up in China in like a month now.