Problem bacteria levels for hands and body are different. Hands are also typically washed for far shorter periods of time than showers.

Standing in a shower for 10 minutes doesn’t mean your armpits are getting cleaned well (or anything else for that matter). I bet most people scrub their armpits for a couple of seconds each. And except for people who shave, armpits are full of hair that interferes with getting them cleaned easily.

The set of people who vigorously scrub their clean-shaven armpits under water for 20-30 seconds each is probably pretty small.