Your example of the engineers makes me wary...
As an engineer, you are legally responsible for the answers you give, and ethically responsible for the lives you endanger by risky answers. So if you are forced to make assumptions about a problem, you make conservative ones! Even after conservative assumptions, if the appropriate data is not available, then factors of safety of 2 are common (meaning 50% of what you could have gotten away with if the assumptions were actually correct).
If this was a real life example, were there soil borings done to determine the soil types? Did the engineers even get to see the site, or were they given dimensions and just told to run the numbers? Engineers will only give you an answer they are comfortable standing behind if shit hits the fan.
You are right that an experienced excavator will be able to give a more accurate answer faster than an engineer, but there are always surprises. For example, there could be a lens of silt that undermines the structural integrity of the soil that no one would be aware of without a soil boring. As a civil engineer, I have seen contractors use there experience and just go at it. Sometimes it works fine...sometimes not so fine, and fingers start pointing. Contractors and engineers both have very important roles, and communication and respect needs to improve between the two in industry. It would be a bad idea to rely on an answer solely from an engineer or contractor.