Actually, the question should be "why do other dairy products spoil so fast?"
If I remember correctly, the spoilage of milk is caused primarily by lactose, which breaks down into lactic acid over time due to the presence of Lactobacillus bacteria, which thrive in that environment.
Butter is mostly fat, and fat does not go bad (it does eventually, but not in the same way or at the same rate - see below) - lactose and sugars together, on the other hand, go bad quite fast. That is also why cream lasts longer than whole milk, and whole milk lasts longer than skim milk. It is also the reason why lactose-free milk seems to last forever - there's nothing to break down, nothing for the bacteria to feed on.
When butter does go bad, it's generally not even for the same reason - it's simply due to oxidation which causes rancidity, similar to a typical vegetable oil.
In short, butter lasts so long with or without refrigeration because proportionally speaking, it doesn't have as much of the lactose that causes other dairy products to go bad.
I can't speak specifically to the shelf life of salami, especially considering all of the factors that come into play (source, type, age, storage, etc.), but when dealing with meat in general, you should never eat it if it smells or tastes "off". Smell is really the most reliable means of determining whether or not meat is still good - far more reliable than the use-by date.
It's unusual for meat to smell or taste sour as a result of foodborne bacteria; sourness is more likely to be due to rancidity of the fats, which is of course equally dangerous, albeit in a different way. Oxidation is very likely if the meat was not properly wrapped and/or stored in an airtight container.
Of course, many people have trouble telling the difference between spoilage due to rancidity and spoilage due to bacteria, and so what you're perceiving as rancid (sour) may really be bacterial spoilage.
Either way, if it tastes off, it's spoiled; don't eat it.
Best Answer
The "best before" date is usually way out. Cut it in slices that you will use within a week. Still Tasty recommends that you use opened, refrigerated cream cheese within a couple of weeks. Very well packed (well wrapped or preferably vacuum packed) cream cheese really should last longer than that. I'd try for keeping a month's worth, refrigerated, in well wrapped packages that you won't open until the last one is gone. Freeze the rest. Frozen, it will lose some creaminess, but will still be good in cooked applications. Again, Still Tasty is a bit (IMO) overly conservative. They suggest (for quality only, not safety), keeping opened cream cheese in the freezer for only two months. I'd allow at least three, if not four. Of course it will stay safe in the freezer indefinitely. In the fridge, just watch for mold, toss the whole thing if it develops mold. You can't just cut off the moldy parts like you can on hard cheese.
EDIT: Just FYI, some places in the US that sell the 48 ounce brick also sell 50 count, 1 ounce packages. Quite a deal for less than $2 more, epecially if the sell-by date is well out. (This picture is from Sam's Club, but I think the other big warehouse store carries the small packs too.)