The US law about turning a child around in the car is one year AND 20 pounds. If someone tells you that "10 months is close enough" or "they weigh 20 pounds so it's fine" - they are wrong. They are encouraging you to break the law. Don't listen.
Your Pediatrician is not a Certified Car Seat Technician, or a Manuel
Pediatricians don't always know everything about keeping children safe. Their job is to keep children healthy. Their job is to treat illness, and prevent death from virus, bacteria, disease, infection, etc. While sometimes they know what they are talking about, it's unfair to expect them to know everything.
If your child were to drink draino, would you call your pediatrician, or would you call poison control? If you think your child has a broken arm, do you take them to the eye doctor? While an eye doctor may be able to diagnose a broken arm, and while a pediatrician may know what to do in the case of draino indigestion, your instinct and experience tells you to pick the BEST source of information available to you. Car seat safety shouldn't be any different.
Laws are not choices
A LOT of people will exceed a posted speed limit, because the penalty for doing so isn't very high. Nevermind the added risk of speeding, nevermind the fact that speed limits aren't just arbitrary numbers picked by people in order to generate revenue. Obviously they are a good way of generating revenue, but that's not the point. If you'll notice, though - many will speed on a regular road, but few will speed through a school zone or an active construction zone. Why? Because the penalty is higher in these areas - usually double. People aren't willing to risk that. Few actually consider that a child MAY run into the road, and going to posted speed limit will give you enough time to stop. I'm sure it crosses minds at some point, but it's one of those "it'll never happen to me" kind of things.
Car seats LAWS exist for a reason. They represent the bare minimum level of safety you can accomplish. There is no law specifically about using the seat correctly, other than which direction it faces. Most states have a law that says you must use the seat according to the manual, but if you're pulling over by police they don't care if the straps are tight enough or the chest clip is too low. People will continue to ignore car seat laws until the penalty for breaking them is steeper.
Stop and Think About It
It's kind of sad really, how we as a society are inconsiderate of the lives of others, and even of our own children, when they get in the way of our own convenience. Heaven forbid we take 60 seconds longer to arrive at work because we didn't want to risk the lives of ourselves or others on the road. Heaven forbid we keep our children as safe as possible, even though they LOOK like they MIGHT BE uncomfortable (but probably aren't).
It also amazes me how many people take basic precautions in every other aspect of parenting. We don't leave cleaning products where our toddlers can find them. We don't let our toddlers play outside alone where we can't see them. We don't let our toddlers go to the grocery store by themselves, or leave them home while we run to the market. And yet we are content to risk injury to their internal organs, permanent paralysis, and even death because "they are happier when they can see me" and "it makes giving them snacks so much easier." Doesn't really make much sense does it?