AC/DC have different costs for voltage converters and per-distance efficiency, so there are some distances where AC makes more sense and others where DC makes more sense; the distance changes as tech improves.
AC mostly conducts on the outer surface of the wire, while DC conducts with the whole cross section, giving you different scaling issues as the current changes.
Under water, AC suffers from significant capacitive loss — the wire acts as one side of a capacitor and the entire ocean as the other.
At certain frequencies and wire lengths you also get inductive losses, though IIRC that affects only RF cables in practice and and the design trans-Siberian railway in theory as no other place even seriously considered having a sufficiently long conductor for the frequency used.
The grounding loops are very impressive. 6 mile loop of buried cable at either end.
Therefore, most new DC transmission systems have a balanced pair of cables, and only use earth return for emergencies
Sadly, it is less safe than AC. Automakers tried to increase DC voltage beyond 12V but it causes sparks that cause mechanical switches to fail. AC sparks are extinguished whenever the voltage wave crosses zero. Ubiquitous power DC needs high power/high voltage solid state switches to become cheaper and more reliable than mechanical ones. Perhaps SiC or GaN transistors will do eventually.
So it’s a nuanced trade off, and if the industry shifts (which tesla is banking on since they’re the ‘leader’) then economies of scale can be reached with higher voltage fuses switches relays etc.
[1] - https://auto.hindustantimes.com/auto/electric-vehicles/tesla...
For example, imagine I want a 3000 volt DC wire to power a portable air conditioner. The air conditioner will be 10 kilowatts, so 3.3 amps. The wire can be thinner than headphone cables (two 0.3mm conductors, +-1500 volts, 150um PTFE coating) if desired.
Obviously, with such a thin insulation, the system needs to be human safe when chewed through by a baby. To ensure that, the current flow through the baby must be under 1 milliamp, or 10 milliJoules through the baby's heart. That can be ensured by tracking the current through each conductor, accurate to 1 milliamp, and shutting off the supply if there is ever more than 1 milliamp unaccounted for (either to earth, or to the other conductor). When the shutoff occurs, it must therefore happen within 1 microsecond (assuming the worst case fault, that is all three amps direct to the baby's heart). That in turn puts capacitance and therefore length limits on the cable - it wouldn't be possible for this cable to be safe longer than ~1000 feet.
TL;DR: It is very possible, with today's technology, to design very high voltage DC systems safe enough for use within a home. However, no hardware available off-the-shelf yet can do this, due to no demand.
3000 volts DC is a "definitely dead" voltage, as opposed to current 110 volt AC systems which are "you'll probably survive" if you use a kitchen knife to cut through the insulation.
Given that you need the protection systems in place anyway, there isn't much point in thicker insulation, unless you like your cord being more cumbersome, heavier and more expensive.
Especially now with USB C. I suspect in 20 years a lot of today's PD supplies will be perfectly good. It's less wasteful when they're that reusable.
When using a transformer, the voltage conversion comes 'free'. Modern electronics makes this smaller, lighter & using less metal (not more reliable, btw ;-)
On the generation side (for example rooftop solar), it isn't a big deal to have 1, powerful, high-efficiency converter.