Why is it very unlikely that aliens could live on Earth?

Question

Why is it very unlikely that aliens could live on Earth? (From a biological perspective)

Answer 1

in my opinion, it's more an issue of physics and economics. To get here from the nearest planet that could even plausibly support life (let alone advanced intelligent spacefaring life) would take a very very very long time. That aside, it's not obvious why they even would! such a journey would be a very expensive and resource-intensive mission and the idea that they would arrive just to disguise themselves for, what, anthropological research? is odd to say the least.

But biologically, on top of the rarity of planets that could support life, it would be a further series of coincidences that our atmosphere would be perfectly breathable for them, that our gravity would be no problem, and that our planet has what they need to eat and drink (or their equivalent ways of taking in nutrients and energy). And all of this without being conspicuous!