Why are companies in general opposed to employees working shorter hours?
Shorter hours means less expenses for salaries. And while a lot of the fixed costs for office space and stay the same, this argument is less valid for companies where a lot of employees work at the client's place. At the same time it could give them a competitive advantage when hiring, which is important in fields where people are hard to find (software development) and recruiters charge a lot.
The issue has become even more important now with coronavirus and the schools having shut down. My employer wants me to use holiday-time to make up for missed hours. I would much rather take a temporary pay-cut and spend the holidays holidaying. Knowing an answer to this question could help me negotiate.
The context is Europe, specifically the Netherlands. This may be relevant, because I believe that on top of salary, employers have other costs per employee that may influence the calculation.
Why are companies in general opposed to employees working shorter hours?
Says who? And what is a problem you are trying to solve? Can you rework your question to describe your situation and ask how to achieve x? Like "I tried to get reduced hours by company refused for XYZ".