It wasn't an exam area, right? It was THE (only) exam area, where a fat demon has been killing all the (13) students of the former water Hashira who stuck him there.

If the toxin requires a significant amount of flowers to be extracted in order to produce usable amounts, those trees will be gone pretty fast and they will lose their protected areas at the same time. They also use the flower, not the leaves, which only bloom for a few months, so it's not like they can keep harvesting them throughout the year.

There is also the issue of the expertise required to process these flowers. If only Shinobu and her team have the knowledge, skills, and tools to do it, it will be difficult to make it standard issue for every demon slayer.