Why is this stretch so hard?

I am pretty average in the flexibilitydepartment - maybe even a little above average when it comes to some hamstring stretches but for some reason certain upper body stretches are just impossible for me. I’m including a photo. I’ve circled the areas that are impeding my ability to get my arms to clasp. My elbows always stick up and out. It blows my mind when all the trainers do this stretch AND THEN BEND OVER while keeping their hands clasped like it’s no big deal. I’ve been trying to accomplish this stretch for around a year now and I don’t think I’ve gotten any more flexible in that area. What else can I be doing to make that area more flexible??