But a strike walk-out is one of all kinds of bizarre reasons one ends up getting scooped. Mice get sick, chemical stocks go bad, collaborators leave for personal reasons, etc. etc. Yes, there's a marginal increase in the odds that one gets scooped during a strike. Truth is, when it comes to transitioning to a faculty position (which is the point of a post-doc position), being scooped is really not that much of a deal. Having the big-ass discovery to one's name can help, yes, but what determines one's chances on the faculty market are a panoply of other factors too --- is the university looking for someone with your research profile? Did they have a funding cut? Is your advisor a famous person known to the hiring committee? etc. Fellows on strikes are acutely aware of the risk of getting scooped every minute that is spent away from the bench, but in the balance, its really not foremost on many folks' minds beyond a point.
So, worst case, people get scooped in the short run. In the longer run, better pay + insurance means far more talent even considering a post-doc position and academia at all. As for whether unions are the way to do it, one-time mobilizations or strikes or nebulous pressure from the public are not reliable and repeatable interventions as and when new issues arise over time. Like, imagine a scenario where a one-time strike gets media attention, gets people more pay but only for a different administration later to roll things back later when the issue is gone. Unions in the US have legal fiat for ensuring lasting changes to labor contracts and can be a pretty effective intervention for these issues.