It is interesting you mentioned the donut spare.

One of the ideas behind that tire was to deliberately make it just good enough to get you to a tire repair but not good enough to drive for a longer time on it.
This was supposed to prevent people from driving around with a flat spare tire in the trunk.

The fact that it took up less space and cost less to make were important considerations too, but not the only ones.
There was a lot of market resistance to them at first, enough that a full sized spare was still offered if you wanted it.