Well this is an interesting thread, as I'm currently in the middle of arranging things for our year 6 cohort.
The play - it is a part of education - learning lines, supporting each other, creating props etc. Education in school isn't just about academia these days, we also do the whole social/emotional side of things now too. We also do a leavers assembly, acknowledging that children have been (hopefully!) working hard at the school for 7 years, getting support from school staff to help them develop. It's a celebration of achievement and recognition for each child's individual successes and how proud we are.
Something we have noticed in school is kids emotional health is a lot different to years ago. Some find it hard to cope with change. Resilience is lacking for quite a large proportion these days, and learning that life moves on, but that it will be OK, is an important lesson we help to teach them through the last few weeks. Learning about transition is a thing in our primary.
Please remember this is their experience, it's actually not for the parents, although some think it is. Granted, some kids don't need it, they have family that supports them through it and have resilience in spades, but other kids do. It's not just about your child.
As for this girl in the OP, it must be very hard for her to watch her friends get involved in the play, and all the talk that comes with it, knowing she is not involved. But I dare say mum feels the holiday is more important to bond with her step siblings. Hopefully mum will be helping her with her emotions and helping her through her disappointment. Empathy goes a long way.