I'm not HR but I did try recruiting for my own startup.
Problem is there's too many profiles applying for roles, and just hiring the right candidate takes up a lot of time. Rejection emails are unfortunately not a priority especially if the first screening itself removes the majority of candidates from pool of applicants. Unless someone uses good ATS and other systems that take care of emails at every step, it's not really worth spending time and money on for a small business.
If someone reaches out though, we do tell them if/why they were rejected. Then again, that's us as a small biz.