Or why I wouldn't be able to control stuff with my mind if the world is mental.
The world is mental even under physicalist metaphysics. "The world" is the simulation our brains make from whatever it is that's "out there" stimulating our senses.
Your eyes aren't window panes that you look out of at "things" beyond them. And, again, I am STILL not talking about idealism yet... We're still straightening out the misconceptions about physicalism. It's interesting that people who want to dismiss idealism rely on naive realism and betray they don't even understand anything about physicalism either.
We do control things with our minds. I conjure memories with it. My thoughts influence my feelings. I can change 'the feel' of the world using spiritual exercises like meditation, et al.
Mistakes to avoid when trying to understand idealism:
1. Naive realism is false. Nobody has any experience at all of a nonmental world, that's impossible no matter what. ALL experiencing is done within your mental simulation of "the world".
2. Physicalism isn't science. You can call it a "science-based" metaphysics if you like, but the same applies to metaphysical idealism. So nobody's defending science against "woo" by arguing for naive realism against metaphysical idealism. Anyone who thinks so has conflated science and metaphysics and needs to sort that out.
3. "Mental" and "soft" are not synonyms.
About #3... A couple centuries back, a fellow named Johnson tried to refute Berkeley's idealism by kicking a rock. "I refute him thus!" he exclaimed and kicked a rock. So, he used a mental experience called "a foot" to kick a mental experience called "a rock" and that resulted in a mental experience of *pain*. What he successfully refuted is the dumb idea that if the world is "mind" (aka, experiences) then it should be "soft" (a trait that can exist nowhere but in experience).
Don't just think about it. LOOK too. Look around at the room you're in. Name ONE thing that isn't "mental"... iow, that isn't a feature of your brain's simulation of "the world".
Here's one of the biggest differences between physicalism and idealism:
Physicalists think their soft, mental, interior world ends somewhere on 'this' side of their eyeballs and skin (their senses). Beyond that is a universe UTTERLY DIFFERENT. It's external to their bodies and so it's "physical" and "hard". Which is a pretty unparsimonious (and dualistic despite physicalism's pretense at being monistic) way of seeing things.
It's like someone standing on a tall hill, looking out to the horizon, and assuming that whatever's past the horizon is utterly different to everything within the horizon.
Idealists don't do that. They assume whatever's past the horizon of what we're able to experience (ie, to know) is more of the same. More "mental stuff".