Because you have to get there and back and there is every possibility of things falling apart very quickly.
I was in the office (in Cambridge) on 25th July 2019, the day of the current record temperature (set in Cambridge

I'll be working from home Monday-Tuesday next week - but I do have an aircon unit at home

I think this is another case where it would be better to just give up trying to run anything at all in most of England between say 1100 and 2000 on both Monday and Tuesday. As I said back when storm Eunice hit, I generally despair at the current attitude of more-or-less giving up at the slightest hint of severe weather, but like Eunice, this looks pretty certain now to be a extremely unusual and very severe event and it would be better to set expectations accordingly. If people get stuck on a train in over 40C heat in direct sun, and it seems inevitable that will happen somewhere, that is very quickly going to become a serious emergency.