For modern high-voltage transmission, capacitive losses matter even at 50/60Hz. ....

That's an overly broad statement. Capacitive reactive losses really matter a lot on submarine or buried cable. Not much of a factor in overhead HV transmission. Think of it like the classic parallel plate capacitor - since that's what we have, just our "plates" are curved away from each other (which reduces capacitance, but let us consider them as flat here). The area (over the length of the lines) is large, yes. But what kills that off so to speak, is a product of two things: a poor dielectric medium (air) and a large distance (many meters) between the "plates".

For "plates" 3cm wide with a length of 1km and a separation of 10m: about 27pF. In other words: 27pf/km.

Formula: (where's my dang MathML slashdot?) C = k * E * A / S where:

C is capacitance in Farads

k is relative permittivity of the dielectric. Equals 1 (for air)

E is permittivity of space, a constant 8.85E-12 F/m

A is area in meters squared

S is separation distance in meters

For that 1km model above the impedance at 60 Hz is 100Mohm. For a 220KV line that is a loss of about 480W/km. Such a line would be conveying power in the few hundreds megawatt range. Not much of a reactive loss there. Different on sub/buried: k is much larger, and S is much smaller (mm - cm distances).