Symbolism, Imagery, Allegory
When the Overlords' ships first appear over Earth, they seem to be like any other alien invaders: They park their vehicles over the major cities of the world, and you settle back to watch the fireworks show.
But instead, nothing happens. That's right—they just kind of sit there and hang out.
This is because the ships aren't going to invade the Earth the old fashioned way. Instead, they are more like symbols. The book even says as much:
The great ships, then, had never been more than symbols, and now the world knew that all save one had been phantoms. (6.11)
All except for Karellen's main ship are simply illusions, holographs meant to trick humanity into believing they were outnumbered and outgunned. In other words, they invade the Earth but they do so through manipulation of mankind's fear, not through an all-out assault.
So the spaceships aren't a symbol of the Overlords' technological prowess, but rather their ability to manipulate through trickery. Instead of a flashy attack, the Overlords are here to subtly guide humanity toward their end goal: the Overmind.