Why is grocery store celery trimmed of its leaves?
You did not disclose your location, but here are some reasons:


It is an opportunity taken by the producer to increase the price. They can sell the product as-is, or process it further and sell it for more (ex. whole onions vs diced onions, the diced onions will be more expensive for the same amount). Sure they need to spend a little more time fabricating it, but they can also sell it for higher profit margins. This is called value added.
Where you live, celery leaves are simply not used as an ingredient. Before you went to Asia, did you ever hear of people adding celery leaves in their food? I've worked in restaurants for five years, and indeed we always cut the leaves off and threw them away. We didn't need to use it. Not once did we have a recipe that called for it.
It is the part of the celery that spoils the fastest. Even if the rest of the celery is fine and all you need to do is cut off the leaves, it is harder for the provider to sell celery with leaves that brown quickly versus selling celery without.