Looking at the ESRB FAQ, you can see some answers to your questions:
Are all games and apps required to have a rating?
The rating system is
voluntary, although virtually all video games that are sold at retail
in the U.S. and Canada are rated by the ESRB. Many U.S. retailers,
including most major chains, have policies to only stock or sell games
that carry an ESRB rating, and console manufacturers require games
that are published on their systems in the U.S. and Canada to be rated
The ESRB system has been voluntarily adopted by several
mobile app storefronts in the U.S. as well, and complete rating
information for all ESRB-rated apps is available by searching the ESRB
website. While certain types of apps may be exempt from ratings,
participating storefronts typically require that non-exempt apps
receive an ESRB rating.
In short, if you want to release your game in retail form, and in some cases in digital form to some specific sites, you'll need to have ESRB rating for the retailers to consider your game for sale.
It's made clear that the system is voluntary. It's not required to sell your game, but it may be required by specific retailers to carry your game.
Not getting ESRB rating limits your options for selling your game, but doesn't restrict you from doing so entirely.