The short answer: These things are temporary states in a multiverse which is always in flux.
The longer answer? Because there are challenges and temptations each of us must deal with. Because we are part of the world and not everyone else in it makes decisions with that in mind.
I was recently thinking about my husband's aunt, the nun, and our last trip to see her. She, and all of her fellow nuns, have made a vow of poverty. They depend entirely on the church to provide for their needs. The facility where she lives is nicer than what most people have. I've been told and read different reasons for this vow: Jesus had no money so the point is to be like him; Money and property decisions distract from spiritual life, so let the church make the money decisions. (So much for the spiritual life of the decision maker, huh?).
As I've said in previous blogs, I admire, and relate to, her commitment to her deity/deities and to her vows.
So why not make a vow of Prosperity? Of Health? A vow, moreso than a petition or spell, is about taking matters into your own hands. It's being active rather than passive. Yes, candle magic is passive. You put together the ingredients, you "charge" it with your "energy" but really what you're doing is making an offering and hoping it is accepted. A vow to accomplish something in the name of your Patrons is a much more involved matter. It is on you to arrange the details to make it happen. Yes, you can still work your work to nudge the universe in your direction, but if I vow to make curtains for my mother's window as a gift, I'm not going to ask her to buy the material and take the measurements for me.
"Prayer indeed is good, but while calling on the Gods a man should himself lend a hand."
What did our Ancestors do? They didn't have the same challenges we face today. There wasn't the same emphasis on fame and beauty and wealth. Sure, these things are nice (maybe except for the fame part lol), but it was more basic: You honor your Gods and Ancestors and the Land and the Sea and Sky. The work done was practical: Glamour to improve your looks/find a good mate, health for the family, a good harvest, protection from the elements, and so on.
Are there vows you have made in haste? Are there vows you have kept/are keeping? Are there vows you would like to get out of? There are ways to do that as well, but that is for another time. Is there a vow you plan to make? Something you have been contemplating?