Precast concrete piles have not "always" been constructed from high strength concrete. Well into the 1950s bridge contractors, in the USA, formed, poured, cured and drove their own concrete piling (typically made from 3 to 4 KSI concrete, reinforced with rebar... not prestressed). Some 1957 photos of this process in a minute.

1) For the same capacity, a prestressed, high-strength concrete pile is smaller and lighter than a precast concrete pile. Note in the photos that precast piles often had octagonal cross sections to lower weight (so a "smaller" crane can be used for driving).

2) Being lighter and "stronger" in bending, modern prestressed piles can be mass produced at a central location, sold, and transported to job sites cost effectively.

The first photo shows a 1957 bridge construction project with the contractor making his own piling.

The second photo (cropped and enlarged from above) shows the piling construction crew at work.

Third photo shows completed precast, octagonal concrete piling ready for driving.