I hear this a lot. I find myself needing to respond to it more often than I have time to, so I find myself writing this in the hopes of not needing to write it again.
I have two separate responses to this. They are independent, orthogonal, and stand alone. Either is sufficient to invalidate this statement in most contexts, but I present both in an attempt to be thorough and to preemptively avoid an argument with someone who accepts one of them but not the other.
First, I want to address "the answer" and "obvious".
In most cases where I hear this feedback, this simply illustrates the average person's lack of care or perspective regarding the issue in question. It's easy for an answer to be obvious if you only consider a small slice of the factors in the question or the consequences of the answer. Someone who is ignorant of those factors or consequences, or simply not mentally equipped to consider them all, will exhibit the Dunning–Kruger effect, thinking they have effectively made a decision that they aren't qualified or equipped to make.
In many other cases, it is plainly evident that different people come up with different, often contradictory and mutually exclusive, "obvious" answers, none of which are "the" answer. This can correlate to the previous paragraph, or it can indicate the use of different ethical or value systems. In that latter case, my mind automatically escalates the question to one of choosing between the two systems in question. The average person's tolerance for cognitive dissonance is something I lack; that doesn't make the answers any less obvious to me; if anything, I can see MORE of the obvious answers than someone who hasn't considered the values leading to their answer and the answers of others.
Second, I want to address my tolerance for causing harm.
I spent a long time in some really unhealthy communities, surrounded by hypocritical people who convinced me that everything I was doing was wrong, that I could never be sure I wasn't doing something bad. Those bad actions ranged from having sex to hugging people to sending messages to sitting next to someone on the subway. I am slowly escaping their influence, and sadly losing a lot of less aware but innocent friends in the process. Along the way, I've become a lot more aware of others' tolerance for unintentional consequences.
When you say that an answer is obvious, one of the things you're saying is that the risks of grave consequences from that answer fall below your threshold for worry/care. I am often, if not always, able to see that answer just as clearly, if not more clearly, than the average person. Where we part ways is in the decision about how acceptable the risks of that answer are.
I spent years afraid to engage in various sorts of actions and interactions with even a 1/1000 chance of causing another person physical or emotional harm. Those previously mentioned people were telling me that even that risk was too much, while also telling me that even attempting to discuss the acceptable amount of risk was itself emotionally harming to others. I have since come to realize that other people operate on risk-of-harm thresholds closer to 1/100 or even 1/20. In that context, it is easy to see how so many decisions would be "obvious" to them when they cause me consternation. The answer is just as obvious to me; I can see it just as clearly. I just don't take/accept it without having to think through an exception to my ingrained rules about risk of harm, or without changing those rules. If you told me an answer was obvious, then I pointed out that that answer had a 10% chance of killing someone, you'd rethink it, right? That's where I am with most of the answers that you think are obvious, albeit not as intense on the consequences.
I hope that reading this has shed some light on why decisions aren't as "obvious" to me as they are to you.