Teacher here. Couple of things:
Summer holidays are a time for children (and teachers) to relax and get some actual rest from the mania during term-time. With the pressures of the new curriculum and ridiculous targets both are completely exhausted by the end of the school year and need time to properly recharge.
Most colleagues I know are ill for at least the first week of the holidays as the body begins to relax. This happens pretty much every holiday and is a sign of just how busy term-time is. That's a week out of the window already and I can only imagine it is similar for some children.
My children don't see me for more than an hour a day during term-time weekdays; they, too, need time with their parents.
Children may return back to school in September with less formal learning retained, but they often grow a lot emotionally during the holidays. The result of some actual time for reflection, relationship building with peers and other adults, time spent at work for some of the older ones and simply new experiences for most of them. Children in the UK are infantlised a lot during term-time, a result of a creeping litigation culture and the need to constantly watch children and interfere in arguments in schools.
I volunteer for 1-2 weeks over summer in a business completely unrelated to what I do. With preparations for GCSE/ A-level results day, analyses and curriculum planning as well as actually enjoying family time, this would not happen if the summer holidays were shorter. I know a fair few colleagues who also volunteer over summer - lost time and experiences for all. My students will certainly benefit from the new knowledge I have gained.
The summer holidays have already been shortened a lot from what they were. A week has been taken off under Baker and been made into INSET-days strewn across the academic year. In order to save money, holidays have gradually been adjusted to include almost every bank holiday going, so the kids now get 190 days of school. With school days potentially getting longer, it is far too much for far too many to handle.
August weather is awful. July is usually a great month weather-wise and the days are still far longer, too. By the time the summer holidays roll around the rain typically returns; I think we had 2 great weeks at the beginning of the holidays and then rain and cold temperatures ever since. I'd much rather they started earlier, or that adjustments for higher temperatures are made the same way they are in other countries (e.g. shortened lessons or half-days at times when temperatures exceed a certain level by mid-day).
Our school building, like so many, is falling apart. Summer is the only real time for rennovations. They do paint walls during term-time, but it can be a nightmare when children inevitably want to touch wet surfaces, trip over equipment in narrow hallways or generally rather watch paint dry (literally) than take an active interest in lessons. Some rooms, like food tech, D&T and Science, would also be unsafe to be around if gas pipes, electrical items or water supplies have to be repaired.
And, to echo another poster, school is not childcare. My children have to be in childcare or school from 7am to 6pm every day during term-time. We just have to make do. The times our childcare facility is closed for holidays we have to make do with either unfamiliar chidminders, grandparents or reciprocal sleepover/ daycare arrangements with other parents. Yes, it is very expensive, but we did factor that in before having children and when considering how many we could afford. School holidays are published many months if not years in advance, so can be planned for in good time, but every year I see posts on here of some to whom they seem to come as a surprise.
Lastly, if you get fed up by the demands of your children (not that everyone here is, but some mums do like to moan about complaints of being hungry, bored or children not wanting to partake in planned activities) time that by 30 and truly try to appreciate what your teachers do for your children. Because we do get this every day during term-time and some parents (by far not all) seem to forget this.