From my limited exposure to designers, I've learned that they think lower contrast strains reader's eyes less than high contrast does. Black on white is thought to be too striking to read comfortably.

Here is a post from 2012 explaining why it's important to use a dark grey instead of a pure black: https://ianstormtaylor.com/design-tip-never-use-black

And I agree. Pure black often looks bad.

Light gray is even worse.