Strongman do bench, see for example Magnus Samuelson training... Benching and squatting!
Magnus is the man and he is allowed to do whatever movement he wants!
Regular bench pressing is just NOT an exercise you find in many Strongman routines. I think when you see them do it in a vid it is simply to show off and give you something to compare yourself to them. Much the way that WSM comps use things like kegs and cars because most people can immediately identify with what it would be like to try and pull a truck or toss a keg 20' in the air. How many times do you see an event where bench pressing really would have helped them power through? Its all overhead and incline. Check it out sometime!