1) Many companies are doing pay cuts. They're not going to pay you for something like this if your base salary isn't even what it should be. It's time to be saving money. Companies are trying to limit spending wherever possible.
2) Most companies are assuming that not everyone is even going to work their regular amount due to children, and other duties. I don't particularly expect people to be compensated more for working at home when it's possible people can't do their job as effectively as before.
3) A lot of companies don't even pay you for commuting time or other similar commitments that are required for getting into work. If they did pay for that, then you would actually be getting a pay cut for working from home since that's time saved.
4) Plenty of companies that already hire people to work remotely, do not pay them more because they don't work in an office. That said, they may pay for your internet since it's considered essential for you to do your job, especially if your role requires decent internet (eg software dev/operations/on-call rotations).
These are just a few reasons of the top of my head.