1 hour of commuting could cost something like 30$ in gas (Europe), vechile depreciation and tolls. Sharing that cost 50-50 or 33-33-33 for a 5-10 minutes delay would yield a 120-180$ / hour rate for the delay period. Very few people could afford to turn that down, it's the average income of an whole hour of work compressed into 5 minutes.
As minor as it is, going to pick someone else up and take them with me is pretty big inconvenience, especially in time.