为什么数据库总是用圆柱体表示？

It's because people view a DB as simple storage, much like a disk. And disk storage has always been represented by a cylinder due to, well, the physical properties of spinning magnetic disks.

I'm reasonably certain that it predates disk drives, and goes back to a considerably older technology: drum memory:

Another possibility (or maybe the choice was based on both) is a still older technology: mercury tank memory:

You may have seen the symbol oriented horizontally instead of vertically, but horizontal drums were common as well:

I always assumed it stood for the round edges of a hard drive platter. The average consumer might not have necessarily known what a Physical Hard Drive Component looked like, so it was represented as a cylinder.

You asked for more pics. I took these at the computer history museum in Mountain View, CA in May 2016. Description for the above image says:

UNIVAC I mercury memory tank, Remington Rand, US, 1951

For memory, the UNIVAC used seven mercury delay line tanks. Eighteen pairs of crystal transducers in each tank transmitted and received data as waves in mercury held at a constant 149°F

Gift of William Agee X976.89

Description for the above image says:

Williams-Kilburn tube - Manchester Mark I, Manchester University, UK, ca 1950

This was the memory in the Manchester Mark I, the successor to the "Baby." It stored only 128 40-bit words. Each bit was an electric charge that created a spot of light on the face of a "TV tube."

Gift of Manchester University Computer Science Department, X67.82