The Men in Black have received a lot of press as their visits are in the consciousness of humans. These entities are allowed to make these visits, as they are in the same density as humans and are not in quarantine. Where do they come from? They are not extraterrestrial at all, but live underground, in tunnels and caverns. Their townships exist in isolation, as they very seldom travel from one township to another, being at risk of exposure to humans in so doing. They fear the Awakening, as humans will then be informed of their presence. Where the eventual outcome of the Transformation, a Service-to- Others world, is not feared by them, they fear the transition. When the Transformation has taken place, this race will continue in 3rd Density, underground. They will then be in quarantine, and will not mix with the eventual inhabitants of the surface world. As they have a steady source of food and reliable shelter that will in the main survive the cataclysms, they fear being inundated after the cataclysms. These fears are not grounded, but nevertheless they have tried to slow the Awakening, in their own way, for these reasons.
The Men in Black have constructed cities underground, structures with multiple stories and tram systems run by electricity, but these are in natural caverns and are not connected to one another unless natural passageways exist. The Men in Black do not generate their electricity from any means that humans are familiar with - rushing water, windmills, steam turbines run by heat generated from controlled nuclear activity or by burning any number of fossil fuels. The electrical energy the Men in Black use is chemically generated, a method learned on their home planet prior to their transplantation to Earth. Their home planet did not have the wealth of fossil fuels that the Earth provides, nor land masses with changes in altitude that allow water to tumble, nor even very much water. Frustration is the mother of invention, as you say, and they worked with what they had. Their electrical energy is not abundant, and would hardly support the average United States homeowner with her many electrical appliances.
If the Men in Black live in natural underground caverns, then how have they escaped the eyes of eager explorers, who pass up no opportunity to discover new underground passageways and caverns? Spelunkers have thoroughly mapped every known crevasse under the surface of the Earth, delighting in every minute of it, as spelunking is an exploration. There are no new lands to discover and explore, as the continents are known and teeming with humanity. Outer space seems out of reach in these days of budget cuts. But spelunking still offers the chance to discover something the eyes of man have never seen. Do spelunkers assume that they have discovered all the caverns and passageways that abound under the crust of the Earth? There are a thousand times more caverns and passageways than they have yet come upon, or even suspect, many far deeper than man has even probed. The Men in Black, being unable to live on the surface, established themselves underground when they first arrived on Earth, in a technologically advanced state. Long, long before humans became proficient at spelunking, the Men in Black were erecting their defenses.
Why have the Men in Black not been found? Look to what clues spelunkers that new passageways exist - air currents, air quality, and sounds such as rushing water. When things are dead still, the assumption is that only solid rock is about. The Men in Black have architect and engineer types who devise ways to test the isolation of their caverns and passageways. Prior to building there, they run a test which essentially pulls the air upward to a vent at the roof. If there are air leaks, connections to other underground passageways, there is air current inward, and this is detectable. Oops, not a good home.
The Men in Black have not been found because of the vulnerability and fearfulness of these hominoids, who do not have the bomb, do not have tanks and bazookas, and in fact do not even have jails and prisons. Why? There is no need. They are not violent, as humans are by nature, but they're scared silly of being discovered by humans. Being no less intelligent than humans, they have spent no small amount of time devising ways to keep from being discovered, like a rat in a hole with no back door. The issue to focus on is not whether the Men in Black have threatened humans, but whether they have acted. When you speak to your dog, whom you wish to impress, what do you do? You act like a top dog! Dogs respond to domination that they cannot overcome by groveling and avoidance. Dogs respond to weakness that they can overcome by either tearing the throat out of a rival that resists or snarling a warning to a rival that turns belly up. The Men in Black have analyzed their co-habitants, and correctly understand what impresses them. Power without the ability to decimate and destroy is ignored. Therefore, threaten.