For the same reason they're sometimes referred to as robots in the media: People aren't always precise and exact in their speech.
Daleks are a post-humanoid mutant creature in a robotic travel casing, they're as hard to destroy as robots, they act inflexibly and robotic, etc. (They're even better at dancing the Moonwalk than the Robot, just like real robots, as Futurama established…) So, sometimes people call them robots, even though it's not strictly accurate.
Behind the scenes, in "Destiny of the Daleks", there's some question as to whether Douglas Adams mistakenly thought the Daleks were robots, and had the characters accurately (as far as he knew) call them that, or whether he knew they weren't and had the characters mistakenly call them that (and even, for that matter, how much of the dialogue he was responsible for). Nobody asked either Adams or Terry Nation before they'd both died, so we'll never find out the truth. But in-story, it can be taken either way, and given that later stories definitely confirm that the Daleks are not robots, it makes sense to take it as the characters being mistaken or imprecise.
Following on from the dialouge from that story, it is implied that the Daleks have relied so much on their technology and logic over the centuries, that they have forgotten that they are organic lifeforms.