The vast majority of the population doesn't do most individual things you could possibly name. There's a vanishingly small set of things you could name that the vast majority of people do without over-generalizing. That doesn't necessarily make them niche.

Most people work, but most people aren't a programmer/barber/chef/construction worker/farmer/etc. Most people listen to music, but most people don't actively listen to <insert any artist below top 100 or so>. Most people use some form of computer daily/weekly, but most people aren't visiting reddit every day, or HN, or a specific youtube channel. Most people aren't a small business owner, but small business owners aren't a niche thing, they are a cornerstone part of the economy.

Add all of these "not mainstream" activity participating people together, and suddenly you have most of the population.