Fake diamonds are a popular choice for gems in a necklace. People sometimes like larger stones in a necklace, and as larger diamonds are going to obviously be more expensive (unless you get low quality diamonds), fake diamonds become an obvious choice.
Part of the reason that some people, who otherwise wouldn’t be interested in a fake diamond, choose to go with fake diamonds for a necklace is because a necklace is something that isn’t worn every day. Some people want a real diamond for their engagement ring because that is something they wear every day, but a necklace might be worn only once or twice a week and so they can’t justify spending a lot of money on a real diamond fr something that only gets occasoinal use.
Another reason people may choose to get fake diamond necklaces is that, since they are cheaper, they can afford to buy more than one style and have a difference necklace to go with various outfits. This provides a larger variety in outfit choices which is very important to some people.
Additionally, if you happen to lose a fake diamond necklace it’s not as big of a deal as if you lose a necklace with actual diamonds.