Because while yes, the death of Jiriya was both a tragedy and a huge loss for the Leaf, the image itself isn't focused on the loss. Instead it's focusing that even after all these years, the Sennin still have a major imprint on the trio's lives. whether it was: -A steady mentorship throughout (Sakura with Tsunade) -Wasn't exactly a good relationship till much later (Sasuke and Orochimaru) -Or even if they unfortunately can't be around to see how their disciple has grown (Naruto and Jiriya). It makes clear that the bond was there for all 3 relationships and has endured.
This picture isn't explictly sad, just has a really sad factor to it.