The way you think is naive to a dangerous and self-destructive degree. A large part of the problem is that we don't know - and, importantly, often can't know - which is bullshit and which is legit. Imagine, for instance, an incident where something bad happened and someone made a ritual to prevent that bad thing from happening ever again. Centuries later and the incident and the reason for conducting the ritual are forgotten, but the ritual is still done and so the bad thing doesn't happen. If we miscategorise the ritual as "bullshit" and remove it, the bad thing will happen.
This exact kind of thing nearly happened in a game called Paths of Civilisation. In it, the civilisation's forest was suffering from a massive tree blight. They devised a ritual that needed doing often so that the forest wouldn't die. Centuries later, the near destruction of the forest by the blight was considered myth and not believed by some. People who were doubting the rituals of the religion in the name of proto-science wanted to test the ritual to see if it actually did anything by ceasing it. If they did, they would've found that it didn't do anything and stopped doing the ritual. The problem is that the effects of not doing the ritual would've become apparent only decades after they'd stopped performing the ritual, eventually dooming the civilisation or at the very least driving it into crisis. The situation was avoided by testing another ritual, one that happened to have immediate negative effects soon after ceasing it, but the point remains - sometimes, it's better to just blindly believe in rituals rather than testing them to see if they work. This is especially true here in 40k where magic and power-through-belief are confirmed to be actual things.