If you set up a new CA, you're not immediately in browser roots until you can prove you're running your CA in a responsible fashion. It's usually easier to pay an existing CA to sign your root cert as if it was one of their own intermediates, and follow their rules to get established before tackling getting approval in browsers. Even after you're approved, you need to wait for users to upgrade to new browsers/OSes/devices (depending on use case) to have your cert included in their trust store.

This even applies if you are the browser vendors, Lets Encrypt was cross signed for the first year or so of its existence.