Why would I have to craft a poison from the venom of a, say, giant spider, to use it? Couldn't I just collect it and at a later date put some on my weapon, for example? What does the crafting accomplish?
There's two reasons I can give you, one from an in-character perspective, the other from an out-of-character game balance perspective:
Poisons aren't as straightforward as you'd think
While we're used from video games that "poisons" are just green-coloured (or whatever) flasks that can be easily applied to weapons, the reality is a lot more complicated. Especially animal venoms are often highly complex organic molecules (or mixtures), and there are a lot of concerns that affects its use as weapon coating:
- Many organic molecules are not 100% stable, especially in air (which has oxygen) and in sunlight (the UV component of which can destroy molecular bonds), meaning that a "raw" poison extracted from an animal might only remain actually dangerous for a few minutes before curdling, decaying or otherwise degrading to the point of being unusable.
- To be useful as a weapon coating, you need to make sure that it reliably enters the opponent's blood stream in a toxic dose upon wounding them. Poisons which rely on different, often slower or more direct forms of delivery, might need preparation to be used as coating. Imagine a poison as thin as water, which would drip off the dagger in seconds. Such a substance would need thickening first to make sure it stays on. A toxic dust, on the other hand, might need to be dissolved in the right solvent to be applicable, and so forth.
- Taking a venom from an animal's venom gland or the poison from its skin isn't often a straightforward process. Could you as layman remove a snake's venom gland without A) poisoning yourself and B) rupturing the gland and therefore spill the venom everywhere instead of getting it into a vial? Poisonous tissues can't be just slapped onto the sword and work, you need to find a process to distil the poison out of the material.
All these steps needed to get from "a frog that is dangerous to lick" to "a vial of goop to spread on your sword" are abstracted together as the Craft check to model the reality of poison extraction being a skilled worker's job, not something anyone can just do on the fly.
Immediate poison loot upsets the balance
Dungeons and Dragons is based on a balance idea of certain resources being rare inside a dungeon and therefore needing to be carried as supplies. While one can turn gold into healing potions (at a vendor) and can acquire gold in a dungeon (as loot), it's not possible to use this mechanism to constantly replenish healing potions in a dungeon because there are no vendors around. To restore the stock, one has to leave the dungeon and find a vendor. The same applies for poisons: They are limited-use, valuable items, where each application is an important decision during the adventure. If a player could "loot" poisons as easily off a spider as they can loot gold off a bandit, any other method of acquiring poison would be overshadowed by this ease. People would not go to a vendor to buy it, but into the forest to kill a few spiders. In addition, the instantaneousness of that looting would mean that replenishment could take place in between fights, sliding poisons very far down on the "replenishment speed" scale. By making poison creation from dead monsters a Craft skill, the restocking of poisons gains a length scale which makes it no longer a no-brainer: If you need to cook the spiders for half an hour to extract something useful, and wandering monsters are checked every 10 minutes, then stopping to cook every spider becomes a dangerous prospect.