Why is simple. If they're monitoring us they probably wouldn't want to affect the result of their observations. That's exactly why people studying nature on Earth try not to have an impact on what they're observing. It affects the outcome and it results in bad science. Or maybe they're shy.

As for how ... we're talking about hypothetical aliens who have crossed galaxies to watch us. Speculating about how they might hide from us when they get here in an HN post is beyond futile. How the heck would anyone know?