I also still believe that transsexual individuals usually still reinforce the idea of gender and even gender stereotypes or generalizations.

So do cisgender people. There are trans women tomboys and feminine trans men, but they're significantly rarer, like they are in cisgender population. Most people will likely only ever meet a few transgender people, if that, and are unlikely to meet the exceptions to the rule as they would with cisgender people.

And, to me, I wonder, “Why cannot these people also reject genders, or be happy with their genders, and just change their sex?”

Imagine all your life, you've worn a red shirt. This is because you have dark hair (or whichever color hair you have), all people with dark hair wear red shirts. You think, whatever, you don't care. Still, you wear the red shirt.

Now let's say there's John. He has dark hair, but would really, really like it to be blond. Now, he can't easily get hair dyes, as hair dyeing is seen as a major, major change to your life. To some, it is even deception, sin, a sign of mental illness. However, blond people wear blue shirts, John knows this. So failing to be able to get his hair dyed, he would at least like to be able to wear a blue shirt.

However, whenever John suggests wearing a blue shirt, he gets major pushback. That's not appropriate. That's not what you're supposed to do. That's deviant. For a time, John wears his red shirt, hating every minute of it, because it reminds him of his dark hair. It reminds him that society apparently cannot deal with him being blond, no matter how strong his wish and how harmless it is to anyone else.

Finally, after what could be years or decades, John cannot bear it any more. He orders a blue shirt. It sits there for a while, or maybe he only wears it privately. But after a while, John puts on his blue shirt, gathers up all his courage and pushes away his anxiety, and heads out into the world, ready for the blowback he may receive.

For the first time in a long time, John feels free. Yes, he may still have dark hair, but finally, he isn't forced to just accept that fact and fall in line. His blue shirt is his signal to the world that he is blond, and he's not going to hide any longer. He's no longer going to pretend. His blue shirt is freedom, taking his life back from everything society has demanded of him.

Much later, John gets his hair dyeing done (after years of psychiatric and physical evaluation), and is now happily living as a blond person. The two of you meet, and get into a conversation. John tells you how important his blue shirt is to him, and you go "Why do you care so much about the color of your shirt, anyway?".