Think about everything that goes into this: they can't just pick a piece of hardware off of Amazon; they have to review proposals for companies to manufacture and support the hardware over their 10-year lifespan. They then have to train their employees on them, develop migration plans, rollback plans, schedule maintenance windows, etc. These migration plans often involve significant changes to CPE configurations, which also need to be planned, tested, implemented and trained for.

It's a huge infrastructure. It costs a lot of money to make any significant change. And you seem to be confusing Ethernet with a last-mile technology; it's more of a last hundred feet technology. A lot of the effort over the last 10 years has been spent moving the telecom-owned equipment closer to consumer homes so that faster speeds can be obtained over shorter cable runs. As the length of a cable run increases, so does interference and you have to dial down speeds as a result (this is even true of fiber, albeit to a lesser extent).