Most Linux server distributions would be expected to be headless.

"General purpose" Linux distributions (not "server") typically would include a GUI desktop.

A GUI Linux distribution feels vastly more niche than a headless one.

Really?

What are some major Linux distributions that are only headless?

What is the market share of those Linux distributions compared to Linux distributions that have a GUI desktop?

If we're counting the millions of servers that run the Internet, probably a lot more of them than GUI desktop Linux installs.