Google, FB, and Microsoft own cables. So I can see that it's data-intensive companies. What does owning the cable do for them? They get to rent it out to other companies?
For example, In the UK if you order a leased line for say 100megs, you'll get some geenric fibre blown into the building. That fibre, and the equipment to run it are all capable of at least 1gig, if not more. However you ISP will charge you a lot more for the full line speed. Why? because they can, that and upstream bandwidth costs more.
If you are interested in point to point, then getting semi decent dark fibre allows you to do DWDM to get 40 gig per strand (most fibre cables are more than on strand.)
a 32 fibre cable (Most of the cost is the physical laying, not the fibre it's self) would give you 1.2tbps theoretical. All for the cost of installing one cable.
Also owning your own cables means you can peer directly with other networks without having to go through a third party. (In the EU, and rest of the world at least, The US have some horrid monopoly system.) This means that the cost of buying bandwidth drops dramatically.
Actually, a heck of a lot more than that - WDM (wavelength division multiplexing) can give you 40Gbps (or more, though cost per bit per second starts to increase substantially) _per wavelength_, with 40 waves per strand (again, more possible, but cost-effectiveness not as good).
So, 1.6Tbps per strand fairly cost-effectively, with more physically possible. That 32-strand cable could carry 48Tbps with a config like this.
Upstream bandwidth isn't really expensive if you're selling to eyeballs (i.e. those who primarily consume services rather than provide them). However, as you said the fixed cost of laying fiber is non-trivial, as is the endpoint equipment to terminate all these waves and do something (route, etc.) with the transported data.
New Sub-sea builds are usually 6 fibers, some large-scale terrestrial builds are up to almost 8000 fibers (euNetworks did this across europe).
Since these repeaters are typically placed every 50-80km, there's a significant number of them in long systems and powering them becomes a challenge. These cables usually operate around 10kV DC at 1-2 amps, and I'm not sure how realistic it would be to go higher. The wire resistance losses are probably already more than 70% of the voltage budget, so you can't really use more current (voltage drop = I^2 R). Higher voltages cause other issues such as dielectric breakdown. Deep ocean cables are actually pretty thin, about as thick as a garden hose, and I doubt you could easily pass more than 15kV. You also have to account for ground voltage shift between the continents which may be as high as 1kV and depends on the weather.