Tradition - as Lighthouse said. It's also part of the big 'fairy tale' they've dreamed about for years and usually nothing to do with religion. Having said that, more and more people are shunning church weddings in favour of far more accomodating venues, especially since the church lost its monopoly on weddings and more civil licences were issued.
https://www.churchtimes.co.uk/articles/2019/12-april/news/uk/church-weddings-fall-by-nearly-half-in-two-decades