The short answer would be: Because even dictators cannot simply do whatever they want.
Let's for example take a look at one of the oldest dictators of history: Caesar Augustus.
Now, technically Augustus was a king in all but name, but couldn't effectively call himself one, because the Romans, since having deposed of their last king centuries ago, despised the mere thought of having a king once again.
Therefore Augustus wisely chose to refrain from crowning himself king and instead carried
the title "Princeps" and proclaimed to be merely the "first among equals" amongst the citizenry of Rome.
Had he chosen to crown himself king, Rome could have potentially fallen into another civil war once again, given the fact that his father had been slain under the accusation of having the intent to crown himself king.
Ironically enough, merely a few decades later the cognomen "Caesar" had become the de facto royal title of the Roman Emperor. In fact, the german language still uses the word "Kaiser", which is directly derived from and pronounced like "Caesar", in the same way that english uses "emperor".
The point is, as said in the beginning: Even dicators with an enormous amount power cannot simply do whatever they want: Even they have to keep up the appearance as in the example above and keep their cronies satisfied, just as the roman emperors had to keep the Praetorian Guard happy, lest they be deposed of.
That being said, the short paragraph about the title "Caesar" was not merely anecdotal, but illustrates how a long lasting dictatorship might eventually drop the appearance and become an open monarchy, provided it manages to successfully establish a stable line of succession, which of course is THE core aspect of a regular monarchy.
The main issue today however seems to be exactly that: Most dictatorships simply don't last long enough and/or don't manage to establish a hereditary line of succession, but in theory it is of course possible:
Should Assad manage to stay in power and have one of his children inherited his position, as he inherited it from his father, and so forth and forth, then one day in the future Syria might be a de facto monarchy, even though the royal title might not necessarily be "king", but something else.
In fact, the better example might be North Korea, where the title of "Supreme Leader" is currently held in the third generation.