Well, people get under invoice, but the dealership doesn't make most of its money on markups. They get their money on financing, finance related add-ons, document processing fees, trades, etc. So, if you come in under invoice, they're making money on you somewhere else.
Just pay attention during signing. Ask them for the interest rate, look at what the monthly payments total up to and how much the car is at the expected full term of the loan. If you add on things like GAP, tire/wheel packages, etc... it all adds up. They love selling "time of purchase only" extended warranty coverage and GAP plans because most people switch cars in 3 years, so it's like free money for them. They'll lie and tell you that GAP is only available through them, etc... but most insurance companies offer GAP and you can usually get it at a cheaper rate. PLUS, that GAP you buy at the dealership is rolled into the price of the car and is also further adjusted by your interest rate. So, they make LOTS of money off of you on GAP insurance. And lots of people pop on those. It's real easy to get a $60,000 base model Hellcat that will cost you $71,000 after everything is tacked on to it over 7 years. Where you might save $1,000 under MSRP, they take you to the cleaners for an $8,000 Chrysler Capital interest cost, which they get a bonus for.
But, overall dealer income is averaged across a body of sales, not just one. They know that X amount of people will opt for Y options at signing... and they need to get Z % of conversions to meet desired revenue targets. So, while you might come out good if you negotiate well and decline all the incentives and pay cash, well, there's 80 other people who didn't. And, when you tell everyone about the great deal you got, the cycle continues.