As wonderful as an idea that it is to be able to go off and talk to people, there are sadly too many out there who would take advantage of such a person for being so open.
As in, it'd be much nicer if everyone just gave strangers lifts in cars, but then malicious people will take advantage of that in order to seclude someone in a car and hurt them and profit from it. Equally, conversations you open up with strangers can indeed result in great friendships, but malicious people could also feign interest and friendliness (particularly if you're an attractive girl!) and then hurt you later when your guard is down.
What you say is very true, but there are so many risks!