Earlier quoted context omitted.
Not just that, Ookla /actively partners/ with the ISPs. Comcast's site is actually at speedtest.comcast.net. There's a financial dis-incentive for them to do so.
Of course this is not all that dishonest. It is just a matter of what you want to measure. For last-mile speed measurement (which is what I personally use speedtest.net most for) using ISP provided local servers makes a lot of sense because it also removes most external factors from the measurement. And really there is no "central point of Internet" which to measure speed to/from.
How many servers on the Internet sit on the same switch on the same data center from your ISP's cable modem head end, though?