Why we can accurately detect stars billions of light years away, but we can’t confirm if we have a 9th planet in our solar system?
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There is a number that tells you “can you see it” called Apparent Magnitude. The lower or more negative the number the brighter the thing to a person on earth. For every 1 lower, the object is 2.5x brighter. For 3 lower in magnitude the object is 2.5×2.5×2.5=15.6x brighter.
On this scale the Sun is -26.7, the Moon is -12.7, and Mars is -2.9. So if you are standing on the Earth looking up into the sky, the sun is 380,000x brighter than the Moon, and the Moon is 9,500x brighter than Mars. The Moon and Mars are only bright at all because they are close to the Sun and the light reflects off them to the Earth. The further something is from the Earth and Sun, the less bright the reflected light will be, unless the thing is bright on its own.
Stars like the Sun are bright on their own. The sun puts out around 10^26 Watts of light power. If every bit of electricity on earth was powering LED lights pointing up to the sky, and there were a trillion Earth copies all doing this, the Sun would still be 100x brighter.
Edit: 100x, not 10,000x
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