How to explain the usage of the present continuous instead of simple in this example?

Question

I was reading "English Grammar in Use" and came across the next exercise:

Ben can’t meet us on Monday. (He / work) late.

The correct answer is "He is working late", but I cannot understand why. There is a grammar written in this book page before:

We use the present simple when we talk about timetables and programmes

and

You can use the present simple to talk about people if their plans are fixed like a timetable

What's wrong with the answer "He works late"? Why cannot we consider that Ben should work late on Monday due to his work schedule, and this is not his desire, but obligation?

Answer 1

The only interpretation under which he works late is possible here is meaning he always works late on Mondays: there's a strong implication that the hearers know this, or should know this.

The sentences you quote from the grammar book simply do not apply to this case, but I'm not clear why not. I'll think about it.

Edit: I think I've got at least part of it. We can use this form when the speaker is expressing their own plan: How about this? I work late; you come and pick me up; then we go to a show.

This is naturally most often in the first person, but doesn't have to be: Here's my plan : He works late. You come here and I'll help you get things ready. Then you pick him up. (Notice that I used I'll in that example: I help would be possible, but felt less natural to me).

It doesn't have to be the wholly the speaker's plan, but it's usually a plan that the speaker is somehow involved in or approves of. (I dont' think this is the whole story: he leaves tomorrow may have nothing to do with me; but it applies to most cases)

But in your example, nothing about the situation is the speaker's plan, so the present simple doesn't work.