I agree with you. Scientists do assume that intelligent intervention has never occurred. This is a result of Occam's Razor, however, and not any anti-materialistic bias. Occam's Razor states that if two explanations are equally good at explaining known observations, the simpler one should be accepted. The two explanations here are that all of the variety of life arose from natural processes OR that all of the variety of life arose from natural processes in addition to one or more intelligent interventions. If there's no data to show that interventions occurred, then, by Occam's Razor, it makes sense for scientists to assume that it didn't. If there were to be found observational data that showed that intelligent interventions were necessary, then honest scientists would have to accept this idea. (I am not saying that all of them would, just that if they are practicing science appropriately, they would.) It remains to ID proponents to show evidence that intelligent interventions must have occurred, rather than just arguing that the idea is not absurd. Good scientists are willing to overturn established theories. The burden of proof is on those who wish to overturn them, however.
The fallacy of this is that you've made an unfounded assumption that this is an either/or problem -- that intelligent intervention and natural processes are mutually exclusive possibilities. However, we already know for an absolute fact that they are NOT mutually exclusive possibilities.
If there's no data to show that interventions occurred, then, by Occam's Razor, it makes sense for scientists to assume that it didn't.
Given that we have practical experience in doing intelligent intervention, it makes an equal amount of sense to assume that it did occur. Indeed, given our human propensity for doing design, Occam's Razor might in fact suggest that design is the better assumption.
The fact that scientists nevertheless invariably assume "no intervention" is evidence of their underlying materialist assumptions.
If there were to be found observational data that showed that intelligent interventions were necessary, then honest scientists would have to accept this idea.
There's an underlying bias there, too, about what constitutes acceptable evidence for design. Based on the way that humans practice intelligent intervention (or design of almost any sort), the proper context for a discussion of design is not "necessary," but rather "desired results." Moreover, there are often a variety of ways to achieve desired results, so "necessity" is not determinative of how a design might turn out -- we would therefore not even expect to find a design to be full of "necessities."
I acknowledge the difficulties inherent in detecting "desired results," but it does not invalidate the underlying truth that they are the driving force behind the process of design. It's worth noting, though, that the "nature's ingenious adaptation" explanation, tacitly relies on "desired results," rather than any argument from necessity.