That's partly because most people are happy with such speeds (or at least the speed/price ratio). At home I have a subscription with 120MBit/s downstream. Speed tests usually give 130MBit/s during daytime and 140MBit/s at night.
Given that, 500MBit/s downstream is the very minimum one would expect from one of the leading Internet companies, probably located closely to one of speedtest.net's nodes.