Ah, so the reason monarchs are the best choice for figurehead is that it's inherited, so on the one hand you have a good heads up about future figureheads (you know there's a good chance from the moment of birth who it'll be) and can ensure they're properly trained for this role, learn some world geography and history for example, practice meeting random people and at least pretending to be interested -- but on the other hand nobody can control who they are, that's just the result of a genetic lottery.
As a result they also have zero democratic legitimacy, which is good because obviously as figurehead they'd otherwise be in an excellent position to seize actual power, which is the last thing you'd want to happen.
As to why prohibited, the temptation is, as you see in the present job of President of the United States to combine the two. Rather than ask people to resist this temptation, just prohibit it up front. These roles are huge asks, in terms both of the skills needed (or at least which ought to be needed) and the burden of responsibility. It would be easy to underestimate the figurehead role in particular, but it just isn't easy to be a symbol for hundreds of millions of people. So, de-risk by insisting on two people for two roles and never merging the role.
This is a cogent argument for having a ceremonial and practical/executive head of state, but doesn't even touch on why a hereditary monarch is in any way superior to having a ceremonial President and executive Prime Minister, as is common in other countries.
The monarch plainly has no legitimacy whatsoever, there's no merit, nobody elected them, they were born into the job.
In contrast an elected President can use the ceremonial position to seize actual power. This isn't even just theoretical, it's an actual problem - or I guess if you've just used it to seize power, a brilliant feature...
Anyway, we don't want that, so the monarch is better.
As a result they also have zero democratic legitimacy, which is good because obviously as figurehead they'd otherwise be in an excellent position to seize actual power, which is the last thing you'd want to happen.
As to why prohibited, the temptation is, as you see in the present job of President of the United States to combine the two. Rather than ask people to resist this temptation, just prohibit it up front. These roles are huge asks, in terms both of the skills needed (or at least which ought to be needed) and the burden of responsibility. It would be easy to underestimate the figurehead role in particular, but it just isn't easy to be a symbol for hundreds of millions of people. So, de-risk by insisting on two people for two roles and never merging the role.