I've thought about this too, because understanding why this doesn't seem to happen more often is part of figuring out how to address the underlying pay gap issue.
My guess is that even assuming a perfectly rational employer (with no unconscious bias), the viability of this sort of arbitrage would depend on women _costing_ $0.70 to the dollar to their employers when compared to men, not on women _earning_ $0.70 to the dollar.
I'm not at all knowledgeable about how this is usually handled in the US, but what I've heard from entrepreneurs in Brazil (where I live) is that despite the fact that maternity leave here is paid for by our social security system, it still ends up incurring in additional costs to the employer, mainly because they need to hire/train a temporary replacement. (Who will most likely be less effective/productive than the more experienced person who took the leave)
Even if the law got updated to grant both genders the same parental leave time (or to allow a shared amount of months to be split by the parents in however way they prefer) it would still require a significant cultural shift to have men actually _choose_ to take the leave. (Anecdotal evidence: I've talked to several local male peers and most of them said they would not bother taking paternal leave even if available, and would rather stay at work)
This sort of market/cost-driven decision making is obviously not good from a moral perspective, and is especially unfair with women that don't want or can't have children, or would rather have children but take a shorter or no leave. However, I also understand that a lot of businesses have very thin margins and fierce competitors, so taking the actuarial calculation approach to estimating how much an employee will 'cost' you might be something they perceive as their only take on surviving.
I'm not sure on what is the best way to (re)align the incentives so that this doesn't happen anymore and the gap is closed for good.
EDIT: what other commenters have pointed out also makes a lot of sense: exercising this sort of arbitrage in the first place would likely require the use of immoral and probably illegal discriminatory practices