It’s not that they can’t, it’s that they shouldn’t. You want to prance around in a dress at 3, whatever, but by the time you hit adolescence, wanting to wear a dress is socially awkward for a boy.
It’s the parents job to PROPERLY guide children and that includes affirmation of the child’s biological sex. Dressing little boys in girl’s clothes is only going to confuse the child. BAD mom.
My girlfriend’s granddaughter wanted to wear everything “boy” when she was little. She’d literally take off her dresses in public she hated them so much. At about age 10-12 she turned into girly-girl. Now she’s a great mom and wife.