For many if not all of the reasons already given, the PA's that are given need to be measured. Most aircraft have IFE systems these days and passengers often rent the facilities. As such frequent and unnecessary interruptions are not particularly welcome.
We usually give a concise route review as part of the PA once established in the cruise. This is combined with a brief review of the en-route and destination weather as well as the anticipated ETA, together with any important safety reminders. On a long flight it is also desirable to provide one or two updates that may include pointing out a local feature of interest or a position desciption. This serves to remind the passengers that we haven't forgotten they are still there. This is usually co-ordinated with the cabin crew so that it takes place at a convenient point in the entertainment programme. We might interrupt for a particular item of interest, and certainly for a safety announcement.
During night flights people do not want to be disturbed any more than necessary. It is often also the case that even during the day, people have been up early or are still tired because of prior travel, and also don't want to be bombarded with announcements. On top of this, as somebody already mentioned, the pilots are always monitoring and these announcements take one of them out of that loop. At times this is necessary, but those times should be kept to a minimum.
It is a fond idea that youngsters conception of geography might be in any meaningful way enhanced by such announcements, but in reality most are more interested in watching re-runs of the simpsons or family guy, or something else they have seen dozens of times before, rather than looking down on Budapest, the Danube or the Dolomites from thirty five thousand feet.