Because you're conflating kernel compile times with an entire OS? Linux userspace compilation can take 6+ hours very easily (try yourself with Gentoo).

At Microsoft, our massive servers churn out nightly Windows image overnight usually 5pm-10am next morning.

The Linux kernel takes 15 minutes. Compiling all of Linux (aka the equivalent of Gentoo emerge'ing e.g. gnome-desktop from whole cloth, which is what this is), would not take 15 minutes.

But that's just the kernel. This is more akin to compiling the kernel plus all packages and applications that make up the operating system.

I clearly rememeber leaving my computer to build during the night, specially when I was crazy enough to experiement with Gentoo.