Why is it difficult to define philosophy?

The question assumes that it is difficult to define philosophy. That's not necessarily true.

Etymologically, a "philosopher" is a "lover of wisdom." In other Words, a philosopher is someone seriously trying to become wise. That's not difficult to understand.

If the concept of wisdom were difficult to define, then it would be difficult to define the goal of philosophy. If the concept of wisdom were not difficult to define, then it would not be difficult to define the goal of philosophy.

Whether or not wisdom is difficult to define depends upon the nature of wisdom, which is controversial. Anyone who is serious about the nature of wisdom needs to articulate what he or she thinks is a sound argument concerning its nature. Whatever argument is advanced, it will be attacked to see if it stands up to examination.

Therefore, it may or may not be difficult to define the nature of philosophy, because doing so depends upon understanding the nature of wisdom. If wisdom is simple, as it may be, then so will be the definition of philosophy. If it is not the case that wisdom is simple, then neither will be the definition of philosophy.

In short, the nature of philosophy is itself controversial. That itself may be sufficient for it to be classified as difficult to define. ----