TAB's comment reminded me of a question I recently had.
Why , when they are looking for a murder victim do they always bring in an excavator ?
They got planted by a guy with a shovel, how deep do they think he went ?
Speed would be one reason. But also if they're not 100% sure "where" they might have to more digging than just straight down.
Plus if I were digging up something I wasn't sure about the condition of, I'd dig around to isolate it then slowly remove the dirt directly on top of it. Sort of archaeology style. Might be evidence on top.
But, hey, all I know about it is what I see on TV shows.