Why are there no car packs in the garage

Unlike Forza Motorsport 5, You still need to purchase the cars in the carpack with CR (First time is usually free). The Day 1 carpack for example are all cars already purchasable as you play through (for the same price as their boring counterparts), but they have a unique livery on them. Purchased Carpacks may be different. I haven't spent enough time in the carshow to be sure ;)

EDIT: It seems there was server issues at launch and car rewards may not have been given out.

Once the servers are stabilized, we will then get to sending out payouts for 
loyalty rewards cars, Forza Rewards payouts, etc.

No. Upgrades and downgrades will not affect your placement in the championship. Opponent Drivatars will always match your class level plus or minus around 10. Even when you enter certain races, Ben (the event coordinator) makes sure and says "pick what you want, I'll make sure your opponents drive similar cars"