They wouldn't have to, if they showed minor conversations such as they did for F4 you would have only seen 4 choices, hell if they showed the average conversation you would have only seen 4 maybe 5 choices.
I didn't say it was devoid of them, just that they aren't common. That's actually a true statement, you see very few conversations with more than 4-5 options, though the ones that are present tend to be large immersive trees. I said nothing about the quality of the trees that are present, merely pointed out they aren't as common as people like to pretend.
**edit: unless you're referring to how a single option would every now and then simply lead to more responses on the same topic line, in which case there is no difference between that and how the 4 buttons probably function. After all Bethesda did have the exact same kinds of branching topics in F3. **