Because its not about the candles. Skull candles are just some fancy melted wax in decorative shapes. They have no inherent magical properties. By themselves, they burn just like NORMAL candles. In fact, the words you chant, everything you do has no inherent magical power. None. Nada. Zilch. It follows the laws of physics to a tea, and has no influence on anything outside its physical system.
Now, there is one part of the equation that DOES have some spooky properties. Specifically, your brain. On top of your brain is running some fancy system software called your subconscious. On top of your subconscious is running another operating system. Your conscious mind.
Now, your subconscious mind can do spooky things. Specifically, if it is emotionally attached to specific outcomes, or believes specific things to be true, events more likely than not tend to arrange themselves in way that create meaning to that person's subconscious. The next question is why, and that's a philosophical question. The answer ranges from the mundane response that meaning is in the eye of the beholder, to a slightly less mundane response about vibrations, and quantum effects and other explanations (which mind you have no serious experimental proof) to somewhat scientific responses surrounding quantum entanglement and the nature of consciousness, to completely fantastical responses saying that reality is a multiverse, we are each living in our own shard of it, and consciousness creates reality. And then we have the matrix people who say we are living in a simulation.
Regardless of why it works, the fact remains that there is a strong coincidence of things happening according to this underlying principle (greater than 50%), and so one can adjust one's own meaning consciously in order to change what that meaning is for their subconscious. The art of creating fairy tail and objectively false beliefs, which then influence the subconscious of the creator, which then cause the creator's subconscious to steer their reality around them is the art known as magic.
ALL religions are fairy tails. All magic is a lie. It is an illusion. An enchantment. The enchantment does become real in *SOME* cases. However, there is external resistance to the enchantment. The magic has very real limits.
limit 1. The less likely an event is, the harder it is for magic to make it more likely. Taking concrete physical actions that give the magic something to work with increases the chances of magic occurring by an order of magnitude.
limit 2. The more discordant beliefs and thoughts about an outcome, the less likely it is that outcome will be governed by your perception of reality.
limit 3. Magic performed on a conscious creature will always come under that creatures cognition if they become aware of it. Aka, counter spells work.
limit 4. Magic affecting cognition is simultaneously the easiest and the hardest magic to perform. Doubly hard if its something your trying to force against another's will.
limit 5. The more people believing in a specific outcome, the easier it is for magic cast along the line of that outcome to come to fruition. This of course is confounded by limit 2