Supposedly there are reasons (for example, efficiency by avoiding the extra weight) and they give you a kit that's essentially a "fix a flat" product, but in the 2 times I've had a flat in a car outfitted like this, what they provided wasn't useful, usually due to the type of damage to the tire.
it sucks, yes, but when coined, the term referred to a platform extracting value from its users by degrading their experience, forcing them to upgrade to a premium plan. your car not having a spare isn't an example of that!
Many of us grew up with cars having spare tyres as a matter of course. Now car manufacturers are extracting value from their customers by degrading our experience (of safety and expedience of repair), forcing us to pay extra for a spare tyre (if even available), or even upgrade to a better car.
Ordinary cars not having a spare tyre as standard equipment any more is a perfect example of enshittification.
IMO its a classic example of real world enshittification