For so very many reasons.
I posted similar on a recent thread, but in a nutshell..
Most of the 'rights of passage' stuff just can't happen if you have not got your full class/cohort in and have to have any kind of social distancing. Show - no time. Residential - can't travel. SATS - cancelled so no wind down after. Leavers' service - can't get people together in large numbers to can't have all parents in anyway.
Seeing friends they are in primary together is a flawed argument if you have to start talking about rotas of kids on days or sessions, or splitting the classes into smaller groups that stay more isolated. And then they can see their friends but not actually go near them....and that's IF you can get all their friends in one group, which you probably couldn't because their friendship groups change like the wind sometimes.
Actual transition activities - a chat by the main high school's teachers to the children going there could potentially happen. But the main visit days won't happen if the high schools aren't open yet. And won't be really meaningful if they are only open to very few kids as they will still miss out on seeing their new classes in their entirety to meet new friends and they won't see what the busy high school actually looks and feels like.
So why keep touting 'transition' as a reason?