Because people feed into sheep mentality. They see that in movies and all over media and think they need to do it too. It's all silly and a gimmick. The jewelry ring industry takes full advantage of this and charge such high prices for rings for such occasions. And millions of people fall for this.
When you love someone, you do it by action and words. No need to kneel down on a knee and get dramatic or display abnormal behaviour like jumping off a bridge. Be your own person and think. Show her affection and stay loyal. These are much more important than an overpriced ring for a nonsensical proposal setup copied from stupid culture.
Also, keep the engagement period as short as possible and save money for real world expenses like buying a house for you both to live together.
It's a relic of an old chivalric tradition, I guess. It makes no sense in a modern society. When a man and a woman marry in a modern society, he is doing her a favour. Despite women's increased labor participation, men earn considerably more than women as a group over their entire careers. Statistically, marriage is a ticket to a higher standard of living for a woman whereas for a man it is a ticket to financial sacrifice. A man should marry a woman only if he wants a family of his own and full well knows what kind of sacrifice that entails.
The kneeling part is stupid. By marrying a woman, a man is agreeing to becoming a beast of burden if he is not yet made it - and it that case she should be the one doing the kneeling. A man foregoes many opportunities by marrying a woman. When a man marries a woman he will most likely stay at the level of achievement at which he already is because very few women these days are content being stay-at-home mothers. If a man has any larger-than-life plans, he can pretty much forget about them as a family man unless he has found a so-called traditional woman or is willing to neglect his family.
Also, with no fault divorce a wife (or a husband) is free to cheat on their spouse without any kind of negative legal consequences. The divorce rate is quite high, almost 50% in developed societies, which means that marrying someone who you are going to outearn by a wide margin even without taking childcare into account, is a financial risk.
Kneeling before a person used to be a sign of subservience - you knelt before the king. Especially if you wanted to ask for something. With courtly manners it was transported through the century. Though a long time a marriage was arranged by the family. Marrying for love and deciding yourself who to marry came into life with the Romanticism in the late 18th century. There men wanted to show a woman how much they loved her etc. and knelt down.
So this tradition came into life and with books and movies was transported into other cultures.