There are several reasons for AC being chosen over DC for distribution. Transformers is a major one - for changing the voltage and for isolation. I suspect that AC generators were more of a driving factor than AC motors - since all electricity is generated, but only some of it ends up driving motors.
A key factor, that gets more important as voltages and currents rise, is switching (in particular, disconnecting). It is vastly easier to turn off AC with a contact switch or relay than corresponding voltage DC.
This is all true, and for the great majority of modern motors and their drives, the AC is first converted into DC. DC to DC conversion would work just as well (or could even be omitted, depending on the voltages in question).
HVDC lines are becoming practical over land too. There are many good reasons why HVDC is better for a grid backbone than AC. As you say, they can be buried - they can even be made with just one line, using the earth (or the sea) as the return line. Capacitance in the cables is a good thing rather than a bad thing. It is easier to connect up multiple sources to a DC backbone - you don't have to worry about phase and
voltage. And you don't have any skin effect - in copper, the skin depth is around 1 cm at 50 Hz, meaning you have to use Litz cables, hollow pipes, bus bars, etc. With DC, your conductors can be any shape you like.
There are, however, three big challenges in HVDC lines. First is getting your voltage up to the high voltage, including matching with existing generators or grids, then there is switching the current, then you must convert to something end users can use (lower voltage DC, or existing standard AC). Improvements in high power semiconductors and control systems have made the conversions a lot easier, though they are much more complex and error-prone than relatively simple transformers. And circuit-breakers for large DC lines are now available, though it is not an easy job to avoid arcing.
Where DC would make a big difference, I think, is for computers - and especially for server rooms. It is /insane/ that you have 220V AC coming in to your UPS, which converts it to something like 24V DC for battery storage and/or supply filtering, then converts it back to 220V AC. This is then passed on to each computer's power supply, which converts it back down to low-voltage DC. A single standardised DC supply voltage would make vastly more sense, and cut out two of the three wasteful conversions.