Like other posters I assume that they have chosen the age groups for a couple of reasons.
Firstly, the early years of education are such important years in terms of outcomes at 18, or even 21. Study after study shows inequalities at this age are incredible difficult to tackle. For every R and Y1 child who has patient and diligent parents at home ploughing through Twinkl and school set work, there may another who is falling further and further behind their peers, and may never regain the lost ground.
Secondly, they cannot learn remotely without extensive parental input, - if you want parents back at work, these ids need to be in school.
It's also important for them socially; I have a Y1 at home and they just don't get remote chats/socialising in the way an older child does.
If the re-opening with these years goes OK, I can see Y10 and 12 being back, and potentially the other primary age groups (maybe on a part time rota type basis) but the other secondary year groups can mostly cope with remote learning so may not be back until September.
In my work I see schools from Nursery to Sixth Form and there is little doubt the Y7 and upwards remote provision is much more effective, even though staff in all age groups are working hard, it's about the maturity of the pupils.