Grad students don't have much budget for voice actors.

That being said, they still make efforts. When I was in grad school, I was routinely asked to do voice overs for videos that accompanied paper submissions by students who had strong accents when speaking English.

Not everyone has loud, presentational voices.

Well... to be fair, if you wanna make sure your study reaches as many people as possible, you must eventually need to improve your presentational skills. Like, you know, we do for everything else. I understand you didn't imply this, but IMHO there is very little pride in wanting to sound introvert forever, intentionally. Now I just hope the parent didn't say that due to misogyny.

Voice presenting for a recording is more of a practical art than we really give credit for. Unless you do it a lot, you aren't going to be very good at it.

Which is the same as with anything else really. You need to practice to be good at something.

Yes, this is another case of the 10,000 hour rule.

Why are all the speakers at tech conferences so bad? And why do good presenters get invited to every conference on the planet?

Public speaking is a skill. It takes work and practice. Most tech folks have neither the time nor desire to do so.

It's copycat behavior. They are imitating other presentations.

Who does? The bad one's or the good one's? Genuine question.

Both.

The good ones snarf ideas from other good presenters when they see something useful.

A lot of bad ones imitate other presentations as the "minimum standard" because creating a good speech takes a LOT of work.

I'm also going to point out that a lot of tech speakers aren't native speakers of English. If you made me, a native English speaker, give a presentation in French or Chinese, I'm guaranteed to be a bad speaker.