Let's jump right into the why.
1. It's a supposedly non position, something followers parrot pretty much more than they say anything else. Just a lack of belief according to them, which is absurd and unlogical in it's on right.
2. It won't be defended because it's supposedly not a position. Any ideology that can't or won't defend itself can't be seriously considered, it's the equivalent to an unfalisfiable hypothesis.
3. They cannot provide the slightest evidence for the position. Literally all we have in favor of physicalism is brain-mind correlation, but materialism has ridden this all the way to the end goal of reduction. After being asked for years by anyone outside their position, still not one shred of evidence has been put forth.
4. The immaterial is self evident, which for any objective thinker discredits the position anyways. Math, logic, the laws of nature themselves, certain fields, and most obviously our own subjective experience.
5. Most will claim the position is default, that we start from physicalism and go from their, despite the fact that this is self evidently not the case. This is an extremely dishonest tactic most groups won't even use specifically because said groups are able and willing to defend their position.
6. The burden of proof is itself a game based in #1, 2, and 3. Again, if a position can and will not defend itself we need not take it seriously.
7. Fideism, faith over science and reason, is rampart in this position, where people will literally deny any valid arguments for gods, will deny the existence of things like the self and math, will deny the benefits of religion, will deny any science not directly supporting materialism, and worse they'll pretend none of it was presented at all. This is done, of course, because the arguments can't be refuted and the position cannot be defended.
I like the number 7, and we have more than enough reasons to not take materialism and atheism in this form seriously. It refuses to defend itself, denies the self evident, has provided no evidence, plays dirty games, and rejects factual knowledge on faith.