The concept of manifesting goes something like this: think positive thoughts, believe you deserve good things, put your intentions out into the universe, and the universe will deliver. It's a philosophy that requires no effort, no sacrifice, and no contact with reality, which is probably why it's so popular.
Write down what you want. Believe you already have it. Feel the feelings of having it. The universe, which is apparently both sentient and deeply invested in your personal finances, picks up on your vibrational frequency and sends you the thing. A raise. A partner. A parking spot. The universe does not distinguish between these requests in terms of difficulty.
You write down what you want. You believe you already have it. You feel pretty good for about a week. Then your car breaks down, your landlord raises the rent, and your vibrational frequency drops to "existential dread." You conclude either that you weren't believing hard enough or that the universe is testing you, which is a convenient explanation that makes the system unfalsifiable, which is the hallmark of all great philosophies and also most scams.
Everyone who manifested a Porsche writes a book about it. Nobody who manifested a Porsche and got a flat tire writes a book about it. You're hearing from the winners and assuming their method caused their success, rather than their success causing their method to seem credible. This is called survivorship bias, and it's the engine that drives the entire self-help industry.
The useful part of manifesting — the only useful part — is that thinking about what you want clarifies your priorities. That clarity might lead you to notice opportunities you'd otherwise miss. That's not the universe responding to your vibrations. That's your brain's reticular activating system filtering information differently because you gave it new criteria. It's psychology, not magic. It works, but not because of crystals.
"The universe doesn't give you what you ask for. It gives you what you work for while also being lucky, which is a less catchy poster."