Because the traditions and trappings of Parliament have all been set up to preserve the myth the the monarch rules the country. But no King or Queen has held any real power for centuries. OK - only the monarch can dissolve Parliament, or declare war, or do a few other things that no-one else can, but just let them try without first being told to by Parliament or by the Privy Council. It would bring about a constitutional crisis of immense proportions. I would suggest it could destroy the United Kingdom as we know it, and probably the Commonwealth too.
The King did not have any power worth speaking about, but he did have something that was every bit as important. He had relatives and friends in high places. Many of the old men sitting in the House of Lords were the King's uncles, cousins, nephews or were related by marriage, and, because they sat in Parliament, they had powers the King did not. So the King could influence events by persuading a like-minded Lord Something-or-Other to vote for or against a particular resolution.
Furthermore, Members of Parliament who sat in the House of Commons would frequently owe their position to the patronage of aristocratic landowners, who would make sure that only candidates they approved of were voted for in the Rotten Boroughs within their domains. Thus, they had to do their masters' bidding if they wanted to keep their Seats.
People rose to power in GB in the same way that people rise to power in US: through influence, wealth and preference. And it seems to me that these factors are influenced more by birth than by ability. Duty and merit have nothing at all to do with it.
As for Mr Washington's honour, didn't he preside over a country that promised, in the Treaty of Paris, to pay all legitimate debts to loyalists, to restore confiscated property, and not to confiscate loyalist property in future, and didn't that country allow debts to loyalists remain unpaid, fail to restore confiscated property and continue to confiscate loyalist property in settlement of unpaid debts? What honour is there in allowing the first international treaty your country signs to be ignored in such a way?
Or was the Treaty signed with no intention of trying to honour it?