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If:
1) Life is likely to evolve on all suitable planets, and
2) There are as many suitable planets as there seem to be, and
3) intelligent, technological live evolves on the average after a few billion years, and
4) life is on average as aggressive a humans, and
5) economical travel between starts is possible
then we would already have been conquered. I’m afraid at least on of these is not true. I suspect number 5.

I suspect that due to your 4: life is on average as aggressive a humans that the average lifespan of a technological civilization is less than a thousand years. YMMV.

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There’s also the possibility that alien cultures either missed evolutionare/revolutionary technology development steps, or we did and they consider Earth a backwater not worth their time.

There’s a great science fiction story on this subject, I can’t recall the name. A highly evolved alien species had developed a technogy that has permitted them to conquor most of the galaxy. The technology is so fearsome that all it takes is a brief demonstration of it and whole planets kneel down in terror and beg to be subjugated and enslaved.

Finally, the sentient life on a cute blue planet orbiting a averageish yellow star in a fairly remote arm of the galaxy comes to the alien conquorers attention. They send their invasion fleet, which lands in the center of one of the planet’s large continental land masses. The alien warriors march out with their terrible technologically advance weaponry, line up in rows ...

And fire a flintlock musket volley.

Upon which they are promptly and efficiently cut to ribbons by the Kansas National Guard.

The story ends with the alien commander being interrogated shortly afte the “invasion” and swearing that he’ll never divulge his races secrets. The interrogator snorts, informs him that Humans have already reverse engineered the interstellar hyperdrive tech and will have operational ships within 6 months, leaving the alien commander to ponder what will happen when humans hit the stars ...

While still looking we have no evidence of civilizations on other planets. Presently we have no evidence to support your position. Doesn’t mean I believe extra terristrial life doesn’t exist, but we haven’t discovered any so far so the possibility exists, though slight, that we are alone. Next question, how do you know aliens haven’t invaded? Perhaps they don’t fight the way we do. As to postulate 5, ever seen a UFO? I have three times in my sixty plus years. Each one I observed exhibited flight characteristics beyond my knowledge of what we currently possess. Are they alien? Unknown. They are simply us identified flying objects. However their flight characteristics are as far as I know beyond our ability. This argues that there is still much to learn about physics.