Attrition tends to happen with the most qualified individuals leaving for other companies.
Attrition is not targeted or evenly distributed (depending on the goals). This can result in what would later be perceived as being lopsided compared to the layoff cut. For example (and purely made up), if part of the layoff was to cut back on marketing of the self hosted instances, then you would be looking to lay off marketing, and developer evangelists. However, if attrition doesn't hit those roles (because who's going to be leaving in this economy), you can have it take much longer to scale back that headcount. Saying to the developer evangelist "ok, we don't have something for you to do, we're going to switch you back to an entry level developer position with this other set of roles and responsibilities with a corresponding pay cut" isn't something that can be done easily.
If you target specific people, even if they do garbage work, somebody somewhere will find something to sue you over. If you do a less targeted layoff, you'll hit some of that target group anyway, and some will leave over their own volition after. Yeah you'll lose some good people but the whole point of this charade is that in management's eyes the engineers and knowledge workers are cogs in a machine, so collateral damage is okay and it'll all come out in the wash over the next few years with new hires.
Attrition is not targeted or evenly distributed (depending on the goals). This can result in what would later be perceived as being lopsided compared to the layoff cut. For example (and purely made up), if part of the layoff was to cut back on marketing of the self hosted instances, then you would be looking to lay off marketing, and developer evangelists. However, if attrition doesn't hit those roles (because who's going to be leaving in this economy), you can have it take much longer to scale back that headcount. Saying to the developer evangelist "ok, we don't have something for you to do, we're going to switch you back to an entry level developer position with this other set of roles and responsibilities with a corresponding pay cut" isn't something that can be done easily.