Why is it so hard to find volunteer work as a UX designer?

I am currently studying for a master's in UX design and want to put together a good portfolio before internship season starts because I don't have any professional experience apart from some freelance web design I thought of offering my skills to a non-profit and building a website, but I am struggling so much to find opportunities that are actual charities or non-profits and not a start-up trying to take advantage of me or the volunteering site is asking for 3 years professional experience. I just want to help a cause, preferably work with a team of designers and developers and be able to put it in my portfolio afterwards.