IMHO it's because the dealers first obligation is to sell these things. If a customer is expecting a delivery tomorrow, the guy or gal ( us) will be pushed back even if you have an appointment. If there are just one or two minor repairs needed you can request to wait an could likely get your repair if all the parts are available - if you have the time to wait.
Most of us don't, and walk in with a list of things, with a appointment expecting to leave it and "hope" you get it back in a reasonable time ( back to my comment about getting pushed back). So the next step is things needing warranty work needs approval by the manufacture or the extended service contract provider. The dealer could wait days for that response (dealers fault .. No). Then the parts can be ordered. Trust me I've been through this numerous times where there isn't stock available ( waited months in the past). Is that the manufactures fault ... No. As they purchase most of their products from various vendors. Once they arrive your unit then gets squeezed in front of someone else waiting, thinking the stars are all lined up all the time.
Manufactures don't always keep tons of inventory anymore. The just in time delivery method is the norm. And dealers certainly aren't going to overload their stock levels on everything we could possibly need for repairs.
That's why we wait