Many do consider changing themselves, and even put forth the effort to do so. But change takes time, effort, patience, and an incredible amount of energy and time. Not just from the individual wanting to change, but from those who know and regularly interact with this person.
If these people do not support the change, this has a negative affect on the person attempting to change, even if it is a neutral reaction. Even worse, if these people treat the person like they have not changed at all, like they can never change, or worse, ridicule and insult said person for even thinking they could ever be better than the [enter expletive here] they "have always been" and "always will be", this kind of treatment can completely wreck a person's morale and self-esteem. It is the opposite of inspiration and positivity, it is negative reinforcement. If a person wants to change, but they receive no positive reinforcement to do so, then the change seems like it will not matter. If this person is insulted and ridiculed for trying by people who, in truth, could use some change themselves (and a deserved slap across the face, and more), then this person definitely will feel like the change is pointless, may be emotionally/mentally devastated, and lose enthusiasm for other aspects of their lives.
Or worse, the person does change, but instead of becoming better, they become worse. Perhaps even aspiring to be a worse human being than the people who gave them crap. There are stories of people who showed great self control when being tormented by others, only to lose it all when they had been pushed too far, broke, and lashed out at those around them, only to lose whatever clout they had and wind up in jail......yet, being pushed to that point by a bully, it's not really their fault.
This kind of BS is also why many people commit suicide. They are pushed, and regardless of how tough someone is, or acts like they are, everyone cracks under pressure and time. Everyone. Doesn't matter who you are.
I know this from experience. The kind of crap I went up against when I attempted to change myself was "harsh", and that is a vast gulf of an understatement. I did eventually change, Freddy. Many have told me I became a better person, due to being more kind and polite than I used to be. But I'm not. Something broke along the way.
It's not always about people wanting to change. It's about one of two other things. Either the world doesn't want to let them change, or the ones in the other side of that fence, the selfish [expletive] who prevent such people from changing....if those people are selfish, greedy, completely morally corrupt, and if this actually works for them.....why would such people ever want to change? Answer, they don't.
It's a messed up world. People don't change to "be good people". If they change, they do so for personal greed, some other kind of personal gain, or for survival to make it through some change that's coming. And if someone out there changes to be a "good person"? Their doing it because being selfish didn't work out so well for them. Still personal, still a selfish change.
And yet, despite all of this, there are still a very rare few truly good people in the world. Out of the thousands of people I have met and interacted with since I was born, the ones I would consider truly good and altruistic, who are good just to be good, I could count on one hand. All the rest, doesn't matter how "good" they act, something selfish still pops up from them.
Call it all paranoia if you like. You might be right. But it's paranoia well deserved from a long life of dealing with too many jerks who claim to be good, not enough truly good people, and though I tried hard to change in this messed up world, this shaped my perspective and world view over time. I'm a pretty dour person some days.
Anyways. My point. It's not always that a person doesn't think about changing their self. Sometimes...more often than you'd think....these people simply are not allowed to change by the people and world around them. They're stuck, being the people they don't want to be, by people who won't let them be anything different. Some get lucky, they pack up and leave. But a lot of people can't do that, lacking the capital, resources, and simply the ability to free themselves and go somewhere else.