Why are modern computer program interfaces often grey nowadays?

Answer 1

What you call "grey" or "gray" are likely the most familiar colour scheme from the Apple GUi and from the unix X-win GUI's.

To some extent a Windows user can affect their own by setting alternates to the defaults - especially the default background content area of windows. These windows options were traditionally called "theme" and the window components appearance are now often termed "chrome".

Some applications in the past merely imitated the OS widgets while some used native OS widgets: this can also affect appearance.

Often "silver" is the dominant color in a style which appears "grey". Other tints, shades and tones are used to achieve 3D appearance.

See https://en.wikipedia.org/wiki/Tints_and_shades

Answer 2

I assume its easier to manufacture and its a color no one really complains about... idk about you but most computers i see are black, silver, or white.