I will admit, I have had this problem all be it I'm getting better at controlling it.

I used to have difficulties with talking in social settings because I had nobody to really properly teach me any better. I would either talk too quietly and people wouldn't be able to hear me or I would speak too loudly. I had a difficult time knowing any better and understanding why I'm wrong.

I was like this because of a combination of introversion, poor social skills, and a mother who wouldn't be inconvenienced to teach me how to do better, instead, preferring to lie to people about me having autism to excuse my behavior.

As an adult, I still struggle with this, although I have gotten much better. I usually have a quiet and rather cold sounding voice because I don't like drawing too much attention to myself, unless I'm around people that I know and feel comfortable around, in which case, I sometimes get excited and start talking very loudly and I have to sometimes have it pointed out to me because I don't do it intentionally.

So to answer your question, people who do this probably were either never taught any better, they probably don't realize they're doing it, or they simply don't care. I just thought I would try to give you a different perspective with my personal experience.