At the risk of having you think me odd for saying so, I enjoy assembling IKEA furniture: it's like LEGO for adults, and I always loved playing with LEGO.
I'm quite good at IKEA furniture assembly too... but I still usually end up with a few extra pieces left over when I'm done.
I think that's a good thing.
I don't know for certain, but I suspect it happens because IKEA chooses to include a few spare parts in each box, knowing its customers might misplace a screw or break a fastener by inadvertently using too much force.
IKEA could choose to include exactly what's required to build the piece of furniture, and not a single screw more. And doing so would likely make the finance people happy in the short term by slightly reducing the cost of each item sold.
Except... it shouldn't.
Because customers who misplaced a screw or broke a fastener would be upset. Frustrated. Unhappy. And they wouldn't blame their own failure, they'd blame "stupid, cheap IKEA". They'd have to return to the store to get another screw, and it wouldn't be a joyous shopping trip. In fact, that shopping trip might even be their last.
That's an idea that should make those finance people very uncomfortable.
I like to think IKEA intentionally avoids all that by simply adding a few extra, "likely to be needed" pieces into the box, just to make things a little bit easier for its consumers.
Those extra pieces aren't a cost, they're an investment.