For me this is more of a problem certain individuals have than necessarily a cultural one.

Sometimes it is a case of habit - in the days before mobile phones becoming prevalent I noticed that a lot of people who talk too loudly on the phone were holding it so that the microphone is nearer to their neck than their mouth. My supposition is/was that people have asked them to speak up and they have done so rather than adjusting how they hold the phone. I wonder with mobile phones if the psychology of not having the microphone next to the mouth also causes people to be louder.
(I have also observed the interesting , if sensible phenomenon, of people holding a phone so that the base [mic] is near their mouth the but the phone then slopes away from the face, nearly flat, with the speaker [top] as far from the ears as possible! This may make the phone harder to hear, but one can just-about see the display while talking and minimises the noise you make to disrupt others.]

My late father reckoned that the way to deal with people too loud on the phone at work was to replace the carpet with a hard surface (to reflect sound back up) and add baffles (vertical dividers dropping down a few inches) to the ceiling to divide the area up into virtual cubicles - these to stop the sound travelling along the ceiling. Having worked in the off office with some of these features I think he's right and it would "cure" some people.
My first job after Uni offered "phone use training" which included NLP techniques to deal with people speaking too loudly or quietly - but these would be no use when not part of the call.

So, going back to the question "Why can’t people be polite and considerate?" I think the answer is because the loud people are the ones who aren't polite or considerate in the first place.

(I was once unable to hold a conversation with a senior manager* at his desk because his boss** was on the phone about 15 feet away talking too loudly for us to concentrate. Said boss then finished his call, walked over to the printer and pressed a few buttons before kicking it - we just looked at each other in amazement.)
* Previous Finance Manager of a small*** company.
** New finance manager.
*** Growing from approx. 100 people to 2 or 300 people.

Sometimes I think the only answer is to be rude back (i.e. start talking at the same volume, even if to no-one) - this actually links to the aforementioned NLP techniques because if you match their volume then start getting quieter they may match your volume.

In the case of the personal finance info the answer is a bit easier - interrupt with words to the effect of "I don't want my personal financial information broadcast to the entire room - lower your volume!"