Didn’t use to be that way, did it...
https://www.instituteforgovernment.org.uk/explainer/calling-general-election
“...Between 2011 and 2022 prime ministers would have had to pass a vote in the House of Commons to call an early election under the Fixed-term Parliaments Act (FTPA) 2011. This has now been repealed and the Dissolution and Calling of Parliament Act (DaCoP) 2022 which has returned the ability to call elections to Royal Prerogative. This is how elections were called before 2011...”
The Fixed Term Parliaments Act, to which you refer, was a short-term political fix by David Cameron. It had all sorts of holes in it from the outset, and its repeal was inevitable sooner or later. We’re now back to the traditional system, in place for centuries, in which the election date is in the gift of the Prime Minister as long as he can command a Parliamentary majority, and no Parliament can last more than 5 years.