One practical reason might be inertia. They have a system which doesn't have any urgent downsides, as the Queen is only formally the head of state but doesn't i

One practical reason might be inertia. They have a system which doesn't have any urgent downsides, as the Queen is only formally the head of state but doesn't interfere with things. Which actually might be nice, as she is a known entity while an elected president might do a worse job. This could change quickly when the Queen dies. T Then there is the effort required to establish an alternative system, both on the design and implementation side. Such a change is a huge step legally. So people might not be in a huge hurry unless driven by need. And then there is of course the sentimental side. There is always a group of people who think the Queen should be the head of state and things shouldn't change. Even in many modern democracies, there are still a few supporters of monarchies. This all isn't of course driven by rational reasons. Just look at some of the sentiments driving the UK out of the EU despite pretty obvious risks for the trade which more and more manifest themselves by now.