Why is there no rendering for railway embankments and cuttings?

The short answer: because nobody has done the necessary work.

The long answer: the standard OSM map style is a complicated beast and the volunteers working on it only have a limited amount of time and they spend that on what they consider important (so part of the answer is that the current maintainers do not consider adding such rendering a high priority item).

You can naturally always volunteer your time for an implementation.

In the specific case there is an open issue https://github.com/gravitystorm/openstreetmap-carto/issues/791 which is however quite old.

As you're in the UK, it might be useful to mention that I've thought about adding some sort of support for embankments (in the first instance flood banks) to this map. What would need to happen is essentially this:

1. Decide exactly which features should have the new tagging. This means looking at the tags that people use, and what combinations those tags appear in. Obviously, the fewer tag combinations are initially suggested, the easier it is to create a prototype.
2. Look at the location in which these tag combinations appear, and choose a rendering that would fit there. For example, here there is a railway on an embankment, but it's the middle track of 3, so there's not much room for detail.
3. Design a rendering (i.e. a combination of CartoCSS properties) that represents the new thing. In this case something based on the bridge rendering or the verge rendering here is probably a good start.
4. Write the necessary bit of lua to identify exactly the tag combinations that you want to capture, but no others, like e.g. here.

If you want to do this, feel free - you can clone both parts of the map style used and tinker to your heart's content - and if you would like any help with these steps, please ask!