Targeted layoffs are tricky to pull off. You can be sued for wrongful dismissal and then you need to show you were not targeting that person by anything other than random. You can easially lay off everyone on a project. Anything where you select individuals is risky if they can somehow argue you choose them because of some status (minority or whatever - even white male is not a status you can dismiss someone on) . Don't get me wrong, companies lay off part of a department all the time - but they would prefer to not do that.
Even if someone quits from a department you don't want to lose people from, you can just transfer an employee from a different department that didn't lose enough people. So this is good enough and someone who quits cannot sue.
Also if someone quits they cannot collect unemployment. Generally governments track how often a company lays off employees and charges higher unemployment rates to those who lay off more people so getting people to quit saves you here too.