Here's why:

* Distance: It's the furthest planet from the Sun, making it incredibly faint and small in the sky.

* Dimness: It reflects very little sunlight, further contributing to its low brightness.

* Position: Neptune is often close to the Sun in the sky, making it difficult to spot during twilight.

While you might technically be able to spot Neptune under ideal conditions (dark skies, no light pollution), it's considered a significant challenge even for experienced observers.

It's worth noting that Uranus is also extremely difficult to see with the naked eye, but it's slightly brighter than Neptune and therefore slightly easier to spot. However, both require exceptional viewing conditions.