I have a suspicion that building 40,000 terabits per second in ~10 undersea links with the rest of the world would be a lot less than 1000x as expensive as building a single undersea link at 40 terabits per second, or one new undersea link every two years of economic pain amidst undersupply. The issue is you couldn't charge end-users 1000x more for it, or even 10x more for it, so it doesn't make any sense for private actors to build: ISPs can't reasonably capture value generated in the economy at large.
From what I'm told, the US built much of its actual fiber-miles count in the late 1990's when projections were that bandwidth usage would grow exponentially for a long time, and we were "running out" (with a high profit margin on each small step forward to add to the supply). Then we figured out how to do wavelength division multiplexing and all of a sudden an existing fiber could carry 100x more information. This led to the existing players going bankrupt, and the "Dark fiber" era, ten years of overshoot before we started digging fiber lines again domestically on a large scale.
This oversupply enabled the Internet in the US to blossom. Last-mile issues were all that existed for the longest time. If we had spent that decade bottlenecked for lack of backbone & backhaul bandwidth, constantly bumping into saturation of the existing links and frenzied construction of new links, the Internet would be a very different place. A more metered place, with a very high degree of rent-seeking by the existing players, and a lot of bandwidth-intensive web ideas killed in their cradles.
Apparently in some other developed countries that joined the Internet scene after this boom & bust, the government made it a priority to provide both backbone and last-mile connectivity on an infrastructural basis without much regard to profit, to even better success than the US.
The current conservative government in Australia has decided that internet is a luxury, not a nation-building project, and scuttled the NBN, a project intended to provide infrastructural improvement to the 'tubes. It ain't going to be a 'national priority' here any time soon.
Which is weird, because business benefits from fast tubes a lot more than consumers do, and usually the conservatives are the pro-business politicians.
Amazon and Microsoft (and others) have Australian based cloud services now, so you'd hope that they might invest in future projects that connect us to the rest of the world.
I know SubPartners currently are trying to build up a few undersea cable networks that connect at various points both domestically and internationally.
Your internet is like your real estate - plenty around and you still pay stupid prices.