It's because not knowing anyone is a catch-22.

You can't start networking if your baseline of knowing anyone is 0. For example, I know of 0 people in the finance industry. The barrier to entry for me to start networking in it is orders of magnitude higher than sending out cold applications online.

This is additionally compounded that networking is extremely dependent on factors entirely unrelated to your actual skills: attractiveness, charisma, social skills, class signaling, quite possibly race and sex.

And you wonder why people don't "just network!".