Before addressing your true question, I just want to say that you can find individuals with almost any characteristic, even non-adaptive ones. Variation can exist for many reasons including genetic (e.g. mutations), or non-genetic reasons such as developmental problems or other environmental causes.
But back the true question:
How can it be explained from an evolutionary perspective that some people have no desire to want children?
Evolutionary psychologists do not perceive the brain to be a conscious fitness calculating machine. To address such a question, it is fundamental to understand the difference between proximate and ultimate causation.
It is true that humans who didn't have any children would, by definition, be less reproductively successful in the past. Therefore, you would expect traits that lead to not having children be slowly selected out.
But you have to analyze this at the proximate level: What caused people in the past to have children? It was not mainly people thinking "I really want a child," rather it was people thinking "I desire sex with that person." What I am getting at here is that it was sex drive, rather than explicitly wanting children per se, that has been selected for.
It can often be helpful to compare us to other animals, especially animals who don't have as powerful cognitive faculties as us. Do you think that all other animals really think that "they want a child," or do you think that by some strange mechanism two individuals are attracted to each other to copulate (It might be scent, visuals, strength, or anything really)?
Of course, I have simplified things substantially. Probably there have been many psychological traits that have been helpful for reproduction other than simply sex drive. Probably also social bonding, and perhaps even the "desire for having children." However, my point here is that the desire to have children is not required, if another mechanism leads to people having children. However, it gets even more complex, because an infinitely strong sexual drive is not necessarily good, either. Being selective in who you mate with can be important as well for ultimate reproductive success.
Next question:
How can we explain from an evolutionary perspective that some "successful" species, choose consciously to avoid parenthood at all?
There is a fundamental trade-off between how much effort and energy you invest in each offspring, and the number of offspring you have. If you have 10 children, you simply do not have as much time or resources to spend on each of them compared to if you had 2 children. This can lead to differences in how many of your offspring survive and eventually have their own offspring (Especially in conditions where resources are scarce). These differences in reproductive patterns were originally explored in r/K selection theory and now in Life history theory. Ignoring parenthood has its benefits in that it allows for more offspring and investing less energy into each.
Now you may ask the following:
But what about after the child comes? For humans, if there is no one there to take care of the child, then it's guaranteed that the child will not survive.
It is clear that humans are a species where parenthood is important, because human infants are so helpless when born and for a long duration. Therefore, you would also expect it to have been selected for that people would invest time and resources in (i.e. to parent) the child when it shows up. You would probably expect this especially for women, because the child can survive without the father, but it is very difficult for it to survive without the mother (in the first few years, especially in ancient environments).
But, in fact, it is very rare that mothers have no desire to protect or care for their offspring after having it. So rare that it does not conflict with the evolutionary perspective. We also know that certain hormones are important for developing social bonds between the parent and offspring such as oxytocin.
