Thats a common criticism but also a misinterpretation of the spells.
For example Speak with Dead answers are brief, cryptic and only what the person knew in life - so if the murder victim doesnt know who killed them, they can't tell you anything useful except for clues that you could gather by searching the area where they died anyway. The corpses is also under no compulsion to tell the truth.
Same applies to Zone of Truth - it compels Truth but not Knowledge and "I dont know" might well be the truth. Marvels Daredevil effectively has a 'zone of truth' ability but that doesnt stop him having advetures.