Over the past 25 years, I've been pulled over maybe a dozen times or so for various things (speeding, headlight out), etc.
Not once have I turned off the engine. Not once have I been instructed—or even asked—to turn off the engine.
I'm white, but you probably guessed that already.
"The reason for the stop is..." -> citation, in-and-out in minutes.
"What the hell was that?" -> citation + wait around while they call their superior officer to figure out what to charge me with.
"Turn the car off and get out the vehicle" -> This is going to hurt.
I bet if we were to look at police training, their flow chart for handling situations is: simple crimes -> write the ticket, move on; minor, but straight forward crimes -> get them to admit to something; you're planning on booking them -> go in blazing.
FWIW, the time I was told to get out of the car and hand cuffed, I had a warrant out due to clerical error with a speeding ticket that I paid, but it didn't get marked as paid. I still got handcuffed and thrown into the police cruiser and only avoided an arrest because the LEO for some reason let me get the receipt from my glove compartment.
That could be where you live. It's pretty standard practice in this area.
I've also been asked to step out of my car several times, and my dad once had guns pulled on him as part of a misunderstanding.
It happens to white people too and it's bizarre to think that it would not?
I do not doubt that it also happens to white people. The point is that it happens to white people at a disproportionately lower rate than it does to non-white people.