Sure, yes, they could come up with a master list and just pull the trigger today. But the odds of sweeping up a huge number of "false positives" grow considerably.
The idea behind waiting is that along with these layoffs are a lot of restructuring work - teams get moved around, some people actually get promoted, newly manager-less teams are absorbed by another team, etc.
When that happens, there will be a certain number of people who might have been underperforming that start to thrive, possibly because of having a better manager, or because they're more interested in their new group, etc. Other people who might have been coasting by and taking up space but getting decent reviews might suddenly be under a better manager and have no where to hide.
But basically, you can do layoffs, restructure, and then in 6 months time re-evaluate.
Typically, barring one-off events, they'll do several rounds here, a few months apart. Done properly, it won't be an every day worry, it'll be a concern 2-3 times over the next year.