Why Don’t Books Ever Adopt the Screenplay Format?

GuyAnswersGirls123

I've recently resurrected my interest in reading fictional literature which I've not had since I was a university student. Yet there's something I'm repeatedly encountering in commercial fiction which I find difficult to follow which is who is speaking in extended sections of dialogue without "business" or unambiguous dialogue tags.

For example, there are lengthy sections in Agatha Christie's "The Third Girl" of just pure dialogue, such as in this excerpt of a conversation between David Baker and Hercule Poirot:

>> “I don’t believe she’d be your type, you know, anymore than I am. Norma’s in London.”
>> “But you said to her stepmother—”
>> “Oh! We don’t tell stepmothers everything.” >> “And where is she in London?”
>> “She works in an interior decorator’s down the King’s Road [...]"
>> “But that is not where she lives, I presume. You have her address?”
>> “Oh yes, a great block of flats. I don’t really understand your interest.”

It continues like this for almost two pages without indicating who is speaking, requiring me to keep an "odd-even" mental stack for which I often get lost mid-way and have to backtrack unless the spoken line makes it so clear who is speaking. She even sometimes introduces a dialogue tag referring to a conversation between two male characters with "he" which serves no function to disambiguate who said what. So I often wondered why it'd be so unconventional to simply do this:

>> David: I don’t believe she’d be your type, you know, anymore than I am. Norma’s in London. >> Poirot: But you said to her stepmother—
>> David: Oh! We don’t tell stepmothers everything. >> Poirot: And where is she in London?
>> David: She works in an interior decorator’s down the King’s Road [...]
>> Poirot: But that is not where she lives, I presume. You have her address?
>> David: Oh yes, a great block of flats. I don’t really understand your interest.

Isn't that so much easier to track?

Updates
13 d
To be clear, I actually love dialogue tags and especially descriptive and colorful ones used quite liberally such as smirked, bellowed, howled, grinned, whispered, laughed, sighed, snarled, stammered, wondered, etc. There's just a threshold when an author omits them and uses, say, twenty lines of dialogue absent any tags or business where my engineering brain that wants to optimize everything -- for both efficiency and ease of use -- thinks the adoption of the screenplay format [...]
Updates
13 d
[...], just in these exceptional contexts, might genuinely be more efficient for readers as it makes it impossible for them to lose track of who is speaking. It does seem to me like there's a lack of cross-pollination there, although I'm admittedly the type of person who, starting from childhood, incessantly questioned the rules to the point of driving all my teachers towards insanity.
Why Don’t Books Ever Adopt the Screenplay Format?
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