As an (ex) climber there's a couple of reasons. I used to use the figure of eight to tie on - until my first leader fall. It took ages to untie the knot, from then on I used the Bowline for thirty years of climbing. Secondly there isn't that much difference in strength between the two knots, about 5% from memory! and in climbing at least, by the time that level of force was being applied you'd be snapped in two! The figure of eight is popular these days as climbing wall (gym) staff can easily check it.
In the tarp world it's the difficulty in untying after loading that's the main issue. My wife's a so and so for it - rather than take the time to learn an adjustable knot she'll do an fo8 on the bight so I'll have spend time at home untying them.
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