The first thing you need to understand is the sheer size of the imperium and how many different groups and sects and cults exist, many of them completely harmless.
Then consider an imperium planet, billions of citizens the upper classes generally paying no attention to what goes on in the underhive.
Not all genesteeler cults take root either, in some cases they will be identified early and burnt out. The ones you mentioned as well as others survive because those who could have stopped it ignore it, are oblivious or just plain ignorant.
To help understand how this happens consider how genesteelers spread in an industrial hive.
A genesteeler will arrive on a planet in secrecy and will move to inhabit the darkest most hidden place. There they will wait until an opportunity arises to breed. The act of impregnating a human also creates a strong psychic link, that female and her immediate family will feel psychically bound to the creature born, they will feel the need to protect it and so will avoid detection and leave populated areas. Hiding away at the bottom of the hive. Over time the number of humans linked in this way will grow. However they still rely on the shadows hiding in the depths avoiding detection.
Then slowly overseers and industrial managers become corrupted, bribes might initially be paid to turn a blind eye. A cult is formed that workers slowly move to, compelled for reasons they can’t explain. Then slowly more people become taken in as they simply follow there workmates and family members.
By this point those that would raise the alarm are either in the psychic thrall of the cult or are dead.
From infiltrating the lower and mid levels the genesteelers can now entrap one or 2 key government officials. People who can help spread the influence. Again this may simply be through bribery and taking advantage of the corrupt, support our cause and we will make sure the factories stay efficient. You can see this I many 40k stories, individuals supporting the goals of Xenoa or Chaos because they simply see a possibility to get more power. When they realise what they where actually supporting they are then horrified (before being cleansed).
By the time the cult becomes well known then those that could immediately do somthing, or call someone that could, are either dead or part of it. At this point the imperium is relying on a wandering inquisitor noticing and taking an interest, or a space marine squad stopping off. You are relying on that one of billions of planets getting noticed, somthing that genesteelers and as a result the humans they corrupt are very good at making sure doesn’t happen until it’s to late.