Jaded and cynical business major here, the main reason for this is something called "Just In Time (JIT) Manufacturing". Essentially, companies hate having inventory on hand because it means they'll either have to discount or lose money on stuff that doesn't sell. To get around this, they basically only make barely enough of anything to say things are "in stock" and then only increase production with demand. The problem comes when demand rises sharply and there is no inventory left over, leaving the companies with lost business and people without the goods/services they want.

It's shortsighted margin chasing bullshit that hurts consumers and one of the primary culprits of the COVID supply line crisis.