That tends to be the case with all technologies though. Initial progress at adding features is much greater (to an outside observer) than later progress. What you probably don't see is the large number of small incremental improvements that are slowly ironing out edge cases, fixing bugs and turning prototypes into something reproducable. I bet if you ask any team involved in those challenges you'd find that the apparently functional cars were constantly requiring fixing and tweaking in a way that the Google cars of today probably don't.