Because the laws over there prevent them from engaging in politics. The King or Queen is a constitutional monarch. UK's laws are made by Parliament. The Royals perform diplomatic duties with other nations, and social outreach to cheerlead for the government and the Commonwealth of Nations, and comfort the people in times of joy and sorrow.
The thought that the racist, narcissistic Markles could reign "better" than Charles and William is mind-boggling. Harry is dumb and henpecked, for starters. Second, she is shrewd and self-promoting but also dumb, and has destroyed relationships with both families.
So?
The Royals can still do their "jobs" and speak out in defense of their nation.
They've failed to do that.
So it is only a matter of time before the House of Windsor will fall from power.
It is likely that Prince William will be the last monarch of the United Kingdom.