Alright, seeing as how you uneducated swine are clearly in need of some enlightenment, I'm going to take some time out of my busy day to hammer in some knowledge so that you'll all be slightly less of an embarrassment to yourselves.
So if belief spawns demons and people by the literal billions believe in a machine spirit, why no machine spirit demon?
First, let's start with daemons. Daemons are not technically spawned by belief. Daemons are shards of the Chaos Gods much like how living cells make up an organism. Said Chaos Gods are formed from the collective emotions of every living thing that has a soul and are empowered but not created by belief.
Everything in the material world is reflected in the Warp. The reflection of a human is called a soul. The reflection of a machine is called a machine spirit. Machine spirits thus differ from daemons as daemons are an inherently different thing - an incarnation of emotion rather than an immaterial reflection of a material object.
Machine spirits also differ from souls in that souls are born from living things inherently capable of emotion while machine spirits only gain things such as personality, power, and free will from complexity and being psychically imprinted on by souls. Said imprinting is done through many ways. Devotion and prayer is a common one, but simply being used by a soulful being is another. A power sword wielded by someone who hates aliens and is used to kill many aliens will be imprinted on by the wielder, making it hate aliens and increasing its power against them even if the wielder does not pray and thank the machine spirit.
Common myths about machine spirits:
1. Machine spirits are actually extensions of/created by a C'tan. FALSE. As explained, machine spirits are created by the psychic imprinting of those around them, not a star god.
2. Machine spirits are artificial intelligence. FALSE. AI is a machine with will while a machine spirit can be considered to be the will of the machine. Similar-sounding ideas but they go in completely different directions which is why AI is abhorrent while machine spirits are holy.
3. Machine spirits are shards of artificial intelligence. FALSE. A shard of artificial intelligence is nothing but a few lines of programming, as significant as a chunk of splattered brain matter and kept out of machine by simply not intentionally including them. Machine spirits are their own thing created by psychic imprinting and are spiritual rather than material programming.