Probably several reasons. To save weight. To cut cots. The spare won't fit over the brakes, etc...
I'm betting you have a little can of sealant + 12 volt inflater in the spare tire well in the trunk. That's what my car has.
Besides, for most people, the spare tire is useless anyway. It would probably be flat in your trunk the one time you need it. Seriously, how many people check their spare?
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