Certainly matter may exist just as certain is it may not exist. Berkeley's unassailable argument is sound that we cannot know.

The idea of the immaterial mind is equally unassailable. Thoughts and emotions are immaterial, even if they are generated by material substance. Given that unassailable fact, Berkeley argues for the simplicity of thinking that everything is immaterial--Ockham's razor. Berkeley admits that we cannot know for certain. He nevertheless provides us with a very interesting conundrum.

Sorry, but I see the question of why things exist to be an empty one. I am not against metaphysics as are the analytical philosophers, but I am reminded of the Buddhist idea that it does not matter how things got this way, it matters what we do with them. Things exist. What we do with them is all that matters.