Typically because you can get better deals on finance for new cars... lots of older second hand cars you'd have to buy outright, people don't own their newer cars.
Buying second hand is also a risk that youll need to pay out for repairs, a newer car comes with warranties, repairs included etc.
It's also a trap where you do that, then at the end of the three years you have no car and no money so start it all over again.
If youve got more money youll buy a car and be able to have it for years, and it becomes far cheaper in the long run
It's like why you'll find people with more money can afford a deposit and to buy a house and be responsible for the upkeep and pay a reasonable mortgage.
People with less money don't have a lump sum for deposit and worry about being able to afford a random 6 grand for a roof so end up stuck in the trap of paying a lot more for rent (thus never being able to save for a deposit)
As discussed things like fiestas, mondeos, and focuses have all been discontinued and replaced by suvs
There was a period where I couldn't afford to buy second hand because of previous experience of spending lots of money upkeeping bad second hand cars that were money pits. I needed the stability of a warranty and a car that was still drivable in 3 years
I ended up finding a second hand did warranties but spent a lot more than if I had just bought of auto trader