Because "financial innovation".
Cars used to be a bit like most things. You get one when you pay the money for it.
Then the financers got in on the act.
Pushing the cost of the car to a monthly payment means several things :
i) It makes the car more affordable in the short term, the tradeoff being that it is more expensive in the long term. This works the same way as mortgages, and means that financers and car companies can push the prices of cars up and skim off money for themselves.
ii) The car company can "capture" your business by making it very expensive for you to exit the plans, so you end up permanently on a plan with them to replace cars.
So paradoxically cars now are more affordable, but ultimately more expensive than they have ever been.
This is a bit like houses where the actual cost of the house is ultimately irrelevant, it is whether you can service the mortgage payment that really determines where you can live.
The benefits are to the consumer, who rather than having to save for years gets instant gratification and the ability to have a new car without having to save for it, at the penalty of paying much more in the long term. And to the financers and car companies who line their pockets with more money that they can skim off, and also confuse the customer as to what they are actually getting into with reams of small print.
Always apply the old addage. If something seems to good to be true, it usually is.
When you go into a car showroom and they push you to go onto finance/PCP, it is because it will ultimately benefit them. Not you.