Question marks missing in J.P. Donleavy's "Ginger Man"

As noted in the following extract, there are instances where question marks are omitted, especially if the sentence implies a suggestion or a request:

Sentences that begin with Why don’t you(I, we) or Why not are primarily interrogatives, and they normally take question marks. A large number of these instances are used with question marks, but some of them are not. The instances with question marks can imply suggestions, offers or requests as well as doubt or interrogative meanings.

The expressions similar to Why don’t you or why not include How(What) about, Can I, Can(Could)you, Can’t(Couldn’t)you, Will(Would)you, Won’t(Wouldn’t)you and Will (Would)... please.

(naosite.lb.nagasaki-u.ac.jp)