Because Netflix doesn't care what your connection to speedtest.net is, they care what your connection to your closest Netflix server box is. A while back, Comcast/your last-mile ISP was throttling traffic to Netflix to get Netflix to pay them. So while Netflix's box had plenty of bandwidth to their ISP, your ISP wasn't using it, intentionally. Fast.com was their response to that, so you could blame your ISP and not Netflix for being slow.

That’s a really over simplification of the issue. Plenty of Netflix edge CDNs are (and always were) ISP hosted. It’s a win-win for both and a complete no-brainer. The ISP v. Netflix argument was always about contract and margin negotiations. Flat rate, usage percentages, minimums, maximums, special plans, cuts, etc. who has the upper hand in the negotiation so to speak. Funnily enough the repeal of net neutrality gave those smaller ISPs much better position in the negotiation with big tech, not necessarily Comcast. The internet discord focused on Comcast and Verizon because fuck those guys. Who is gonna argue in favor of Comcast or Verizon? But the real winners were thousands of smaller regional ISPs.

I meant, acquire the speedtest.net domain and point it to servers inside Netflix' farm.