As far as I know, neither Angabnd nor Thangorodrim appear in any print copies of The Silmarillion. I did manage to find a fan-made map of the northern-most region of Beleriand, which shows Angband in relation to Dorthonion and Anfauglith:
I'm not aware of any out-of-universe reasons for not including it; from an editorial perspective it seems like a strange omission, for exactly the reasons you note.
In-universe, it's because The Silmarillion is meant to be Bilbo Baggins' translation of ancient Elvish history (Particularly ancient Noldorin history). The maps are similarly Bilbo's recreations of ancient Elvish maps. So the reason is basically that the elves never got around to mapping that region.
It's not hard to see why they made that decision. After leaving Aman the Noldor were pretty single-minded in their pursuit of the silmarils, so exploration of other lands wasn't high on their priority list. This would explain why official maps are most detailed around Elvish settlements.
There may also have been political motivations. Obviously the Noldor weren't on friendly terms with Morgoth, and didn't often venture near his lands. There's not much incentive to put your enemy on a map, especially when everyone knows exactly where to find them (Just look for the three massive volcanoes that dominate the northern skyline).
The simplest explanation, though, is the same one that explains why Mordor is so poorly-charted in maps of Middle Earth in the Third Age: nobody knew a whole lot about the Iron Mountains at this time. As far as I know the only beings who went to Angband and lived to tell about it (Before the Battle of Unnumbered Tears, at least) were Beren, Luthien, and Hurin (Father of Turin). Not many opportunities for cartography.