Why do deaf people shout!?
Firstly, I agree that deaf people often do speak loudly. Interestingly, if they are able to wear a hearing aid, they do then speak much more quietly, and you can often tell whether or not they are wearing their hearing aids just from their own speech. So my conclusion is that deaf people speaking loudly are instinctively doing what they imagine other people need in order to understand them.
It is also worth mentioning that people do tend to mirror each other's behaviour to some extent. So speaking in a loud voice can - again instinctively rather than intentionally - be a tactic to get other people to speak more loudly and hence be easier to understand.
Now assuming that the question was actually an expression of exasperation rather than a search for information, I do sympathise to some extent. Living with a deaf person can be taxing. For ways of handling the situation, see the page on preventing deaf people from shouting.
Disclaimer: The information on this site is for a lay audience and I cannot be responsible for errors or omissions. The views, strategies, advice and suggestions etc are based on my personal experience and are not necessarily appropriate for anyone else. They should, hopefully, stimulate individuals to develop their own strategies.