I run a school, and I teach programming to children from poor households as part of my church's outreach program. 's answer may apply to adults, but I deal with young children and teenagers on a daily basis and the struggle applies to them as well.
It's not just about having energy. It's about the attitude that the environment engenders.
Many of the kids I teach live in slums. People pee on the stairwells. Graffiti adorn the walls. Common areas are littered with uncollected rubbish. And it's the people living there who are doing it. There is a soul-crushing sense of hopelessness and apathy that permeates environments such as these.
And it's not just about trash, or the pee, or the graffiti. Hardcore poverty has a distinct sub-culture. Imagine if your social circle looked like this:
Your parents are working two or three jobs and they're still struggling to make ends meet; or they might be unemployed and scraping a living on odd jobs and welfare. In your circle of friends and acquaintances, having and holding on to a minimum wage job is an achievement to be celebrated. Some of your friends may be involved in drugs or alcohol or gangs or a combination of all three. If you try to keep your head down and study, you get ridiculed and labelled with terms such as "nerd" or "boring".
All these factors and more contribute to an overwhelming sense of helplessness. Many of the kids I teach are resigned to a lifetime of menial work or crime. It takes a lot of work to even get them to acknowledge that there are other options, and even greater effort to get them to work towards those options. These kids and many more like them will grow up inheriting these self-limiting mindsets, and will likely never grow out of it. A large majority will be the next generation of hardcore poor.
The psychological effects of poverty are very real, and escaping them is easier said than done. Which is why education is as much about psychology as it is about moving information from point A to point B.