At least in nato, nato standard ammo and magazines make easy logistics between allies. As far as weapons, well, if theyre all 5.56 and feed from stanags, might as well use the lightest, most modular, most ergonomic, and lightest recoiling 5.56 that uses stanags...it just so happens that the ar15 is all of those things. Further simplified by huge proliferation, parts availability, common practices for services, and 70 years and countless fortunes spend on developments and updates, ar15 makes huge sense for nato nations.

Its what the soviet bloc intended to do before they realized that the ussr is was a silly government with silly government things happening so they couldnt actually get every bloc nation to build the exact same gun, so parts didnt interchange.

Now there are some outliers like the hk416 being in use, but there really are only two patterns of ar15 in wide circulation: hk416 and milspec..and after a quick google search the brits went with knights so that bullshit bcg system (with whatever rail and muzzle device). That really does keep spare parts issues to a minimum (you can drop a us m4a1 bolt into a canadian c8 or a us m27 bolt into a german g95 or whatever the french call their gun.)

Now keep in mind that there are a few nations that still use home grown system (l85a3 for the brits and aug for the anzac).

But by and large, all of nato agreeing on a single type of service rifle with the same ammo and mags makes great logistic sense.

Now onto camo, multicam was developed to work "good nuff" in most places. As a nation that fields an army that may need to operate abroad, it makes sense to have a camo pattern that works "good nuff" in most places. You can issue things like overwhites or jungle cammies if necessary, but in the effort to cut costs, issues one camo pattern to rule them all to all of your troops just makes good logistical sense. Otherwise, youd need to issue troops 6 sets of cammies, pcs, pouches, bags, rucks, etc etc etc. why do that when you can issue 1 set of "good nuff"?