They don't know which 20% needs to be cut. That's why they wait until something unrelated to their business internals (market downturn, other companies are cutting) happens to try.
I remember seeing somewhere an analysis that layoffs do not improve medium or long-term stock performance for companies performing layoffs. But if layoffs are coordinated sector-wide they can have a big impact on employee compensation in the sector by creating a reserve army of unemployed.