The events that led to the conquest of northern Africa by Arabs took place what, 1000+ years ago? You can hardly argue that “Arabs are colonialists” in modern times by judging 1000+ years old historical events, in the same way you probably wouldn’t judge modern Greek people as colonialists because they speak an Indo-European language and Indo-European languages are spoken all over the continent of Europe and much beyond.

Arabic, Arabian would refer to the geographic region of the Arabian peninsula. The dialects are on a spectrum, neighboring countries might understand each other reasonably well, but not countries further apart. Syrian, Lebanese and Palestinian share a lot of features, same goes for gulf countries, Egyptian is pretty distinct but well known outside the country due to its output of movies, music and tv series, Algerian and Tunisian are pretty similar, Moroccan’s kind of its own thing, Maghrebi dialects also include a lot of vocabulary from Berber languages, which won’t be understood in other regions. Finally, in most countries, local dialects are not taught in school but rather Fusha, or modern standard Arabic, which is the language you’ll hear on the news or read in the papers, and is common to the entire Arabic-speaking world. People don’t speak it day to day but usually understand it well and can communicate in a mix of that and their dialects if they’re speaking to someone from another Arabic-speaking country.

This is 100% amateur guesswork, but maybe the geography is part of the answer here? Norway is a bunch of extremely jagged coastline opening on to the fairly cold and empty North Sea, and most of the rest of it is equally jagged mountains, so it was probably easy for communities to be relatively isolated most of the time and therefore wind up speaking a little differently to the guys in the next fjord over. The Maghreb, on the other hand, is right on the Mediterranean, which has been one of humanity’s busiest and most travelled areas for thousands of years

Norwegians seem lo love linguistic about as much as Finns do. Because of this enthusiasm, people have been making extremely detailed dialect maps for decades. My guess is that these two countries are the exception and the rest of the world focuses on funding other types of research.