The problem is the Queen owns lots of land. Lots and lots of it. George III traded the rents for that land to pay off his debts. Each Monarch reaffirms that deal even though I think the terms only applied to GIII. If the monarchy goes away, the title to the land still stays and Parliament loses the rent and tourist income and also has to deal with a powerful and maybe aggressive landlord.
Being King or Queen there is not worth a warm bucket of spit (powerwise) kinda like the U.S. Vice Presidency.
Not to mention in that system of government the Monarch is the equivalent of our Constitution - the Monarch is what gives every government institution it’s legitimacy to exist and its powers are delegated authority from the Monarch. Every government official swears to protect and defend the Queen just as ours swear to protect and defend the Constitution. This institution is not merely a ceremonial office and the Monarch is not just a figurehead and it would not be simple at all to do away with it.