I think it's not prepared. More like a willingness to be open to possibilities. Preparation assume you know something good is coming and can prepare now to face it. Those kind of events are probably not that life changing. Of course there are exceptions.
But best things in life come from opportunities that you cannot prepare for in advance. Just willingness to accept it.
Perhaps pedantic, but "prepared mind" includes your meaning. Prepared may mean being equipped with a skill that meets the moments, but flexibility, acceptance and optimism are also skills of a prepared mind.
It's why the quote has survived since 1854.
> "dans les champs de l'observation, le hasard ne favorise que les esprits préparés" ("In the field of observation, chance favors only the prepared mind")
I don't think it is pedantic at all. A prepared mind is exactly what is needed to take advantage of opportunity. Prepared mentally denotes a state of mind where you are actively looking for opportunities and that allows you to notice them. It is an attitude, really. Not unlike being a Stoic is an attitude on gratefulness of life and acceptance of death.
I used to think the same (and still have regrets about missed opportunities, ie sleepwalking them). But then I think that I'm probably disregarding some other opportunities that I did take and just forgot about them because "For it falls out That what we have we prize not to the worth ..."
But best things in life come from opportunities that you cannot prepare for in advance. Just willingness to accept it.