TSA knows who you are way ahead of time when you put your secure flight information in, and maybe it checks the information when your boarding pass is printed. Airlines close check-in so they don’t have unexpected people showing up during boarding. But if you get rebooked on a different flight or something, they can check you in at the gate whenever.
Also, United and Delta let you check in 30 minutes prior to departure if you don’t have bags.
TSA knows who you are way ahead of time when you put your secure flight information in, and maybe it checks the information when your boarding pass is printed. Airlines close check-in so they don’t have unexpected people showing up during boarding. But if you get rebooked on a different flight or something, they can check you in at the gate whenever.
Also, United and Delta let you check in 30 minutes prior to departure if you don’t have bags.