As noted by another commenter, we're "social" animals. So consider "strength" in the context of the social dynamics of the mating group. That is, in some group

As noted by another commenter, we're "social" animals. So consider "strength" in the context of the social dynamics of the mating group. That is, in some groups, literal physical strength is going to have benefits, and be sought after because of how it can be physically applied. In other groups, the aesthetic of being physically strong brings along social standing, which is attractive for its own reasons. Etc. The point being, cultural norms of beauty, whatever they are, correlate to the other desirable traits and outcomes (money, power, safety, stability, etc) that humans want, in that context.