In my previous company there were preferred part lists and those were bought in huge quantities in advance and dynamically stocked up.
I have asked that same question many times. However...........
Depending on how the accounting and tax structure is set up, inventories have very significant financial implications.
Don't ask me to explain them, I don't fully understand them myself.
If you order something, you dont pay it ASAP. For example, you order a million resistor for your doodad, you only pay for the resistors 60 days later. In that 60 days, you assemble the doodad, and send it to a customer, and get the money for it. From this money, you pay the resistor supplier. It is called company credit.
If you buy a large stock of components, which you dont use it right away, you need to pay it before you got payed for it. It means that the company actually has to have that money, or get it by other means, investors, bank credit, etc. It doesnt sound like a big deal, but it is. It means that a small company can do big business. If you spot buy on Farnell, not only do you pay potentially 10-100 times the price for the component, it also means, that only big companys can do small business.
Of course there is also the possibility to have a lawyer sitting on meetings with suppliers, and write contracts, that are very expensive to break. I've seen a PCBA company assemble a board with loss for years, because breaking the contract would cost more. Big players are doing this, basically when you order, you also state a penalty for late delivery is x% every week. Sometimes it is even just setup in a system like SAP, if you didnt get the parts, they get a penalty invoice with no human interaction. Of course production will see it, but that is another story.