Because otherwise you are talking about something without knowing what you are talking about.

If there has been a long history of philosophy in China and you are simply unaware of that, then you may easily make untrue statements about philosophy qua philosophy being a Western thing.

That is why.

I mean--if you want to only talk about western philosophy that is fine, but admit that it is what you are talking about and do not attempt to say it is the only philosophy out there until you looked around for another sort.