Hey Mynus, good question.
The reason is that commander deck building concerns itself with colour identity rather than just colour. They're are similar, but slightly distinct.
As a general rule, a cards colour is determined by whatever is in the top right of the card, which is why
lands don't have a colour as such.
Colour identity is defined by any mana pips present on the card, which is why different coloured activated abilities etc. can make a cards identity different to it's colour.
Basics, no matter what they look like, functionally essentially have "tap: add {colour} to your mana pool". They don't always display it that way for aesthetic reasons but they're so intrinsic to the game it doesn't matter. From a rules perspective, that's what it says.
That means their colour identity is the same as the mana they produce. Same for dual
lands/guildgates etc.
Lands like
exotic orchard,
command tower and even fetch
lands can be played in any deck. They have no mana symbols and so no colour identity.
Edit: a bit of further information.
It's worth noting that Commander (and it's variants such as brawl or oathbreaker) is the only format that cares about colour identity. The idea of a cards colour identity was brought about specifically for commander.
Also, only mana symbols present within a cards frame and rules text count towards it's identity (which includes casting cost and what not). Any mana symbols that appear in reminder text do not count towards it's colour identity, which is why it is OK to run
Crypt Ghast in mono black. This is something I've tripped up on myself several times so thought it was worth mentioning.