I'm going to suggest two features that might lead ethicists to advocate some risk-averseness (or at least caution towards taking risks).
The Naturalistic Fallacy
One thing to keep in mind for this discussion is that we should not confuse what nature has us do with what we ought to do (nor however should we make the opposite error and assume what we should do has nothing to do with nature).
In your question, you note that social biology explains that we are by nature risk-averse. This does not by itself make it good or bad. There's two reasons. First, nature is full of all sorts of bad things and good things relative to us (Arsenic, for instance, and sugar). Second, we still need to have ideals about nature to know how things should work. For instance, without an idea of how we think nature ought to be, it's hard to know whether there's anything wrong with a bear ripping apart a seal or a fox eating a baby.
Aristotle, the Mean, and Risk Taking
First, I would suggest there are good Aristotelian reasons. For Aristotle, the task of ethics is to be virtuous, and in good part this means finding the mean between extremes. Aristotle defines the good in terms of human flourishing. Thus, the right extent of risk-taking is the extent that maximizes our flourishing.
Taking large risks all the time (at least to me) appears to be something that leads on the average to negative outcomes. Conversely, taking no risks is also not a good life strategy either. At the same time, the consequences for taking large risks and failing in general outweigh the consequences for avoiding risk. Consequently, the mean in terms of risk-taking should be closer to the side of not taking risks. At the same time, this does not mean it should be wholly without risk.
I take it that for Aristotle risk-taking is both a task of phronesis (pratical wisdom) and a virtue related to courage (but not identical because only courage in battle and political courage get full consideration in Nicomachean Ethics BK III).
For Aristotle, having a vice in terms of risk taking would be bad because the outcomes for human flourishing are bad. Similarly, having a vice in terms of taking no risks would be bad, because (a) that's impossible and (b) the nature of life on this fragile rock is such that we can rationally conclude certain risks are worth taking.
Contemporary Kantians might also be able to avail themselves of something like the above account by understanding flourishing as the flourishing of rational agency. (I personally don't think Kant would go for that, but there's some issues that make it possible related to the complete good vs. highest good).
Utilitarian (Consequentalist) Reasons for Avoiding Risk
For the utilitarian, the point of ethics is to maximize happiness. For consequentalists more broadly, it is to maximize some good or minimize some bad thing (or set of goods / bads). Consequently, it's just going to be the same as the risk calculation assuming we have some calculation of the potential harm and potential gain.
If we don't act for the best, we are doing wrong (though an important ambiguity in classical Utilitarianism is whether we act rightly when what we do accomplishes the good or when what we aim for (regardless of its achievement) is what we reasonably expect would maximize the good)).