With that attitude there wouldn’t be murders anymore. But there are, so we have to assume there are limitations on what either Speak with Dead can do or how available it is. Our investigator isn’t a spellcaster, or lieutenant Hanthorpe already cast it to no effect (standard procedure at BGPD)
How to get around things like Speak with Dead is a bit of a topic in D&D; considering how prevalent Speak with Dead is in the EA it is a little odd they don’t address it here.
It's the same reason in the D&D movie that they didn't make Doric (the Druid) or Edgin (The Bard) spellcasters - even though both are primary spellcasting classes - it breaks too many challenging scenarios when you can just magic out of them.
In the case with Speak with Dead though you would just have to remove the persons jawbone or head to take it off the table. I think it's unfair to ask us to accept the rules of this world and the logistics of how magic affects day to day life - but then also ask that we ignore them when it's not convenient.
Wouldn't it make more sense to write - although it would be more challenging - within the established rules of the world and how they would affect everyday events?
Like in the case of a murder you would have people on the force that would use magic to determine what happened, just like we would take DNA samples and fingerprints.
Otherwise - we are not dealing with Baldur's Gate but a snapshot of Victorian England and this is actually a Sherlock Holmes esque scenario.
