Why are chip designers called "triangle pushers"?

They are called "polygon pushers".

polygon pusher: n.

A chip designer who spends most of his or her time at the physical layout level (which requires drawing lots of multi-colored polygons). Also rectangle slinger.

Early masks for the creation of layers on an IC were created by a photographic process that involved exposing the original photographic plate through a mechanically controlled triangular aperture. Hence triangle pusher.

The light source was fixed above the aperture, the plate was moved xy underneath. The point of a triangle was that additive it could give any orthogonal geometry required.

There were no laser printers back then.

In computer graphics, a poly-pusher (short for polygon-pusher), is a system that uses a brute force approach to simply draw as many polygons as possible, as opposed to more intelligent systems that try to figure out things like which triangles are visible, or ray-tracing. Historically, brute-force has always won when compared to more complex systems. The winning philosophy seems to be, "do the simplest thing possible, as fast as possible.". Thus most modern-day graphics cards are "poly-pushers".

Not heard the term used for a hardware engineer, but it's possible that someone who believes in simple, fast, hardware, might be called a "poly-pusher", or maybe a "poly-pusher-pusher" :-)