Because not all evidence is equally valid. Anecdotal reports, for instance, are not reliable. People can be fooled. There are optical illusions, mass hysteria, hallucinations, observer bias, etc. As far as observer bias, people often pay attention to things that support their predetermined viewpoint and ignore things that contradict that viewpoint. People tend to think coincidences aren't the result of the laws of chance, but are rather signs of something special related to them personally. Miracles are likewise never proven reliably to have subverted any natural laws, indeed churches tend to demonize people who find out they are simply the result of a leaky pipe or something. So, different people accept different things as evidence, but those people accepting unreliable evidence are wrong to do so.