Why Unity uses reflection in order access to MonoBehaviour messages methods like Awake, Update, Start,...?

Wouldn't be slow to use reflection? Why it doesn't leverage other approaches like Template Method? It could simply define the methods as abstract in MonoBehaviour base class and force the subclasses to implement it.

So as you see in the answer my assumption was wrong. However, the
answer can be useful for users hence I kept the post.

It doesn't

How Update is called

No, Unity doesn’t use System.Reflection to find a magic method every time it needs to call one.

Instead, the first time a MonoBehaviour of a given type is accessed the underlying script is inspected through scripting runtime (either Mono or IL2CPP) whether it has any magic methods defined and this information is cached. If a MonoBehaviour has a specific method it is added to a proper list, for example if a script has Update method defined it is added to a list of scripts which need to be updated every frame.

During the game Unity just iterates through these lists and executes methods from it — that simple. Also, this is why it doesn’t matter if your Update method is public or private.