Partly I suspect this is selection bias. You say you see so many luxury cars go by. But if you're looking for them, you're going to notice them. Have you calculated the actual percentage? Do they make up 50% of the cars that pass a specific point in a specific period of time? Or just 10% if you really counted?
You say you live in Baltimore county, Maryland. That's a relatively wealthy area, so I'd expect the percentage of luxury cars to be higher than the national average. You'd likely see considerably fewer in the backwoods of Mississippi.
That said, some people who own luxury cars can't really afford them. I'm reminded of a wonderful TV commercial I saw recently where a man is showing off all his material goods, he talks about his big house, and his swimming pool, and his fancy car, with a big smile on his face, standing tall, and generally looking proud and happy. And then he says, "How do I do it?" And suddenly his expression changes to complete despair, he slumps down, and says, "I'm in debt up to my eyeballs." It turns out to be a commercial for a debt-counseling service.
Some people put very high value on owning a fancy car and are willing to sacrifice on other things. If having a big fancy car is more important to you then, say, having a nice house or the latest computer or a big screen TV or dining out more often or going on more expensive vacations or whatever you have to give up to get the car, well, that's your decision. Personally I don't care much about a fancy car, I just want something that gets me where I want to go. And I've always figured that with an expensive car, you have to constantly worry about getting in an accident and damaging or destroying it. If you put your money into a big fancy house, at least houses rarely collide with each other.
Personally, I make a nice income too. And I have a $500/month mortgage and zero car payment because I drive a 2003 pickup that I bought with cash. But I have two kids in college and I'm trying to get them through with no debt, that's where all my money is going.