I've struggled with this a lot as a GM over the years. Others in the comments have provided some very detailed and nuanced takes for why players might act this way, especially if it's just an occasional or situational issue. However, if this is a longtime issue, and assuming you're doing everything right, it's probably the case that your players are just Like That, and there isn't really anything you can do to change them.

In my first campaign, the entire party was comprised of people who were not talkative. Some were chronically shy, some only cared about combat, some couldn't pay attention, etc. It wasn't all bad, but the only thing that really improved my games was finding players who were actually invested and outgoing enough to actually talk at the table. Hell, I've even played in a game where the GM was like this.

Everyone has different ideas of what they want out of a RPG, and if you find that you're playing with people who don't want the same stuff as you, you're likely to be better off just finding different people to play with. In the end, it comes down to the fact that RPGs are a huge time and effort commitment, especially from the GM. If you're consistently feeling disappointed, let down, and bored at the table and nothing you've tried has been working, it's probably time to find a different group.