For years there have been exhortations from every level of the "style" press saying that floral dresses are over, yet women is the UK still wear them - OP, you are not the first person to make this observation.
It's because they are really practical. They don't ride up and show your knickers, you don't get too hot, you don't feel cold in an air-conditioned office, you don't look out of place at work, on the school run, in the park, at a wedding, down the pub, out for dinner, or anywhere else. If it rains, they don't stick damply to you (unlike, say, jeans). If it's sunny you won't get burned.
Also, on a grey day, they're quite cheering - easy to enjoy tasteful black linen when you have 4 months of reliable sunshine, but in an unpredictable island climate you need clothes that make you feel summery when the weather doesn't play ball.
Partly this is just groupthink, in the same way that skinny jeans persisted for years after they were over according to the fashion pages. But it's also because they tick a lot of boxes, just not the fashion ones.