Perhaps it was because MS didn't want manual PATH alterations to be easy or they suspected too many people would have made their program only work if there was a manual alteration to it.

The DOS prompt being terrible for 20 years might have similar justification. In 1995 MS would have many developers who would have just shifted their DOS apps to the command line if they could have gotten away with it.

The OSX finder may have been made similar ideas in mind.