Actually, that's not how it works. Because Harry had the title Duke, Meghan has to have the title Duchess. Same with Kate Middleton. Their husbands aren't next in line to the throne. Prince Charles is known as the Prince of Wales because he is the next in line for the throne. That is why Princess Diana was a princess when they were married. Camilla Parker Bowles isn't a princess, because they changed the rules in order for their marriage to occur. When Charles becomes king, it'll be likely that Wills and, therefore, Kate's titles will change to the Prince and Princess of Wales. However, it may not happen exactly like that considering all the rule changes that have occurred since Charles and Camilla married.
Not defending them, just explaining how it works. I actually can't stand the royals, as an Australian, and wish we could become a republic. I am so disgusted in how they protect Prince Andrew