I would say it would be unusual not to have double glazing these days. It has been a thing for decades and there are grants available to make your property more energy efficient.
That said there are a lot of a) old buildings that are protected and so can't have the windows replaced easily/in an economically viable way (e.g. each window needing to be hand-crafted in wood to non-standard sizes to look identical bto what was there before), b) a lot of just properties nrented out to single people who rent a single room etc and the landlord simply doesn't care/bother, c) houses that were lived in by someone for the last 30/40/50 years where basically no maintenance or decoration had been done - they've died and someone else had moved in.
That said there are a lot of a) old buildings that are protected and so can't have the windows replaced easily/in an economically viable way (e.g. each window needing to be hand-crafted in wood to non-standard sizes to look identical bto what was there before), b) a lot of just properties nrented out to single people who rent a single room etc and the landlord simply doesn't care/bother, c) houses that were lived in by someone for the last 30/40/50 years where basically no maintenance or decoration had been done - they've died and someone else had moved in.