Working conditions are awful enough that companies can afford a high turnover rate, but can't efficiently fill too many positions at once.
If people quit two or three at a time, management just redistributes work around, with extra pressure to deliver.
However, if everyone leaves at once (or goes on strike), there's nobody to redistribute the work to. Even if you could hire replacements on the spot (which you usually can't anyway), you can't just say okay, you're hired, now start working. New hires need company- and department-specific training, they need to coordinate with other people (who would also be fresh hires) and so on.
Modern companies don't run solely on management directions, they also run on an enormous body of practical knowledge and experience which bridges management intention with practical action. Without this, nothing works.
"Infinitely replacement" is a comfortable management myth -- it works only as long as it happens gradually. You can "infinitely replace" each part of a sailing boat, but if you try to infinitely replace all of it at once, everyone just falls into the ocean because at some point in the replacement effort there's no bloody boat. Same here. A 50-people company can afford to replace virtually everyone over the course of one year, as long as it's four fresh hires/month for 12 months. Fifty fresh hire in a single month while also keeping the shop open, on the other hand, is a no-go.