Because the English royal family is the most famous and important royal family to ever walk the face of the earth? And if I remember rightly, they have sovereign over 4 American states, Canada, Australia, Zimbabwe, Hong Kong, Pakistan, Iraq, South Africa... Pretty much 60% of the colonies we used to own. This may be irrelevant in the name of power, but that isn't the point... The point is that they influence.
And the fact that the royal wedding brought well over £3 billion worth of profit in merchandise and advertising before the wedding even began. So it was the biggest boost in our economy for a while.
And royalty is a symbol of national pride.