I thought hen and stag dos were an ancient tradition but most websites date their origins to the 1970s.
If anyone has been to an Indian or a Turkish wedding they will have seen that these start several days before the actual nuptials with lots of lavish ceremonies where the bride and groom are celebrated in single sex parties and traditional rituals and given beautiful clothes and there is fantastic food and dancing.
Western girls probably just wanted a slice of that.
On googling the subject, I came across the theory that the "hen" name came from the traditional painting of the hands and body with henna, which many Eastern traditions practice. I was at an Indian wedding where a specialist artist came and painted the bride and many of the female guests the day before. The men had had separate celebrations. The groom was a Sikh and he had had a special turban binder come and bind his turban. The whole wedding party did look fabulous on the day.
Pre-wedding celebrations sending off the bride and groom into their new lives were certainly always a thing, for example with the ancient Greeks. They just seem to have to have gone out of fashion in Western society, possibly due to the puritan movements of the 16th century.
In Germany the tradition is a "Polterabend" where the couple celebrate together with all their friends and the guests smash crockery outside the house and the couple have to sweep up the shards. This is reminiscent of the crockery and glass smashing traditions at Greek and Jewish weddings. However my eldest son and daughter-in-law had separate hen and stag dos organised by their friends. The boys did go to Hooters, but it wasn't really their thing (I was led to understand). The girls had a Chinese meal and went to a karaoke bar, but all of this was in their home town.
The brothers Grimm describe a "Polterabend" in their dictionary (19th century). Then, as now,, this involved merry-making, games and pranks (usually at the couple's expense)
I tried to have a get-together the evening before my wedding but I hadn't told many of my friends about it and anyway they had mostly moved away by then, so it was basically an awkward afternoon with cups of tea at the house of my one friend who was still there and who was going to be a bridesmaid.