One reason, beyond cart theft, is aesthetics.
A grocery store at Bayview Village, an upscale mall in Toronto, uses this system to stop cart travel outside the grocery store parking garage. Mall management considers carts trashy and that they otherwise bring down the appearance of the mall. This was one of the conditions when the store opened in 2005. Their cart policy may have changed 20 years since.
I provide a counter-example. In the city I live in, there is only one regional grocery chain, and they always have a bagger push your cart to your car for you. Was sort of annoying when I moved here 15 years ago. The parking lot has no corrals for carts, because the bagger always takes the empty cart back.
These carts have, for the last couple years (I don't remember when, exactly), the locks on the front left wheel. It can't be to "disable the carts if someone tries to take them out of the parking lot". That isn't an issue. Though I have not seen it in action, I suspect that if I tried to take a cart through the exit without somehow moving it past whatever device deactivates the lock would have the cart lock up and start skidding (though with the lock being on the front, anyone should be able to just pop a wheely with their body weight and keep on trucking).
That said, I don't claim that these are effective at loss-prevention, but sometimes those jackasses get crazy ideas in their heads and won't be dissuaded by common sense and reality and all those other naive things.
I frequently get so much in a grocery trip that even my body weight wouldn't be enough for a wheelie...
Right, the real reason isn't to stop theft, it's to avoid the optics of store-branded carts being left around and save management the hassle of retrieving carts from nearby properties.