Why? Because they are his son and daughter in law.
I am an unrepentent republican but that doesn't mean I don't have For reasons I can't discuss I've come to have some well-founded respect for at least some members of the family, including Charles, his second and current wife (but never his first) and his late mother, as human beings rather than the almost god-life creatures they are officially depicted as, and I began to realise that these people are just as much prisoners of the institution as anybody else. The courtiers, all people of a very conservative and traditionalist, disposition make the rules, and the family has to live within the bounds they set unless they can slip the lead for a while. Anecdotal evidence abounds of the late Queen being easily distracted from protocols when somebody piques her interest, or of bantering with a group of squaddies just off manoevres she'd given a lift to on the way back from an off-duty trip to the races, or of chit-chat with the cinema manager hosting her at a royal perfermance. She and the avowedly republican Robin Cook (the best PM we never had, in my view) got on like a house on fire because of their shared love of horse-racing. It was known that she wanted her visitors (as for investitures) to feel comfortable and not have to do anything that felt wrong to them, like bowing and curtseying. But the palace insiders insist on maintaining the protocols within the privacy of the palace, and when Meghan was told that she was expected to curtsey, not just to the Queen but to her brother and sister-in-law but in her husband's absence to just about everybody else connected with the family she was furious, and so would I be. It looks and sounds like a deliberate humiliation from the custodians of protocol, a vicious dig at somebody who was clearly "not one of us". For Meghan was not only of mixed heritage, she was brought up in a very different culture in a foreign country; she is not evil, she is an American, a Californian even, and a Southern Californian at that, raised to be emotionally open and candid, physical and unstuffy and undeferential. It didn't sit well with the stoical, stiff-upper-lipped traditions of the court but it's not her fault, if indeed it is a fault.
And before we get carried away with notions of centuries of tradition, it's always good to remember that most of the frippery and flummery associated with the royal family is not much more than a year old, beginning with the elaborate arrangements for the funeral of Queen Victoria. Victoria was an unpopular recluse for much of her reign.