A good photographer will take great pictures with whatever they have at the time. This is because they have the experience and usually professional training.
However, professional wedding photographers still use big cameras (DSLR's) why?
There are many reasons:
The image sensor in a phone is about the size of a pea. The size of the sensor in a DSLR is 36mm x 24mm which is about the third of the size of a credit card. This is a big difference. The effect of this is image quality. This enables the wedding photographer to print poster size pictures without any loss in quality.
The professional wedding photographer will have a range of expensive professional lenses which maximise the quality of the photographs.
The professional camera gives the photographer full control over the image
The camera will usually have 2 memory card slots, one of them being used as a backup
The photographer will usually shoot in RAW. This is a file unlike one from a phone (jpg) which is packed with digital information to maximise image quality.
I think all photographers would like to carry smaller, lighter cameras especially when they are shooting all day at a wedding. I have done this so many times and it does take it's toll after a few hours.
Technology is moving on and some photographers are moving towards mirrorless cameras but I don't ever see a time when a phone could replace a professional camera.