Although, at a high enough zoom, Earth could also be a crescent.
Planets closer to the sun than you can be in a range of places relative to you and the sun, and so you can see parts of it that are illuminated and parts that are in shadow. Venus and Mercury can both be seen as crescents when viewed from the Earth
When you're looking at a planet further out than you are, you only see the part of the planet that's facing the sun because that's the same part that facing you. So from Earth, the planet Mars looks like a disc.
This is so cool.