I have never had my ID checked at the gate on a domestic flight, only by TSA at security or at the ticketing counter when checking bags.
Edit: And here's the TSA notice clearly stating the policy - https://www.tsa.gov/travel/security-screening/identification
I'm sure there's some nefarious thing you can do where two people collude to swap boarding passes after security, but I don't believe it's worth the effort for airlines to try to prevent that. Maybe something with discounts or mileage accumulation; but it'll be a big pain if there's anything out of the schedule.