The same reason a lot of people "lease" their phone, which is effectively what a contract is. 45 quid a month for 2/3 years. Then get a new phone and contract.
Getting a mortgage means you "own" a property you can't afford in outright cash terms.
So a car is no different, you pay your £300 a month for a brand new car and after 2/3 years, it goes back and you start again. What's the problem?
While I agree that leasing a car isn't necessarily always a bad idea, I can see a few difference/problems in your various examples.
Firstly, if it were true that finance always has a non-zero costs, and it mostly is, then borrowing is always more expensive. But no-one can afford to buy a house out of one month's salary, whereas many could buy a phone or even a car. So housing is different just because of the size of the amount.
Secondly, with both phones and cars, I think people struggle to analyse value-for-money and buy something shinier and more expensive than they would, if they were paying cash. That's true even if they can afford to pay cash up-front. I think the easier payments do result in people signing up to worse value-for-money.
Especially with phones, there's a competitive market in phones sold directly (not contract) and there's a competitive market in mobile contracts to cater to those phones, so what possible advantage can there be in rolling the two things into one contract, other than to pull the wool over the customers eyes or get them to sign up for more than they really want/need.
I have a prejudict that anyone who has a contract phone is not the sharpest at managing their money. I'm probably being unfair, because there's a somewhat competitive market in contact phones. Having said that, it's fragmented by the fact that different people have different telecom needs, making each groups market small than than the markets for phones and contracts in isolation from each other.