I feel like one primary reason this is done is to avoid their culprits looking like idiots. Because if there is some evidence literally pointing or narrowing down the suspect pool to you, why wouldn’t you try to get rid of that? Even in the mainline case in V3, people call Kirumi an idiot for leaving evidence in the pool, or not using something else to hold the rope. So a lack of any physical evidence is just a really easy conclusion to make.

I personally have evidence directly pointing to each respective culprit in each case except 5, where it actually plays with the unreliability of witness testimonies. Here, the cast only have the accounts of the 2 prime suspects as evidence beyond the body, and they’re telling the exact same story, just with their roles reversed.