If you look at the list of Standard Languages, you'll see that most of them use Dwarvish as their script:
- Common: Common
- Dwarvish: Dwarvish
- Elvish: Elvish
- Giant: Dwarvish
- Gnomish: Dwarvish
- Goblin: Dwarvish
- Halfling: Common
- Orc: Dwarvish
(While that's 5e, I found a similar chart for 3e.)
The Forgotten Realms Wiki calls this script Dethek and explains some of its history, but I can't see how the explanation there relates to the Standard Languages at all. I also know next to nothing about DnD in-universe history.
Is there an in-universe explanation for as to why these languages all use the same script?
I don't particularly care where the explanation comes from as long as it is from some DnD canon where the premise is true. I don't expect too many settings to have an explanation at all, which should keep this from being too broad.
Best Answer
Dragon Magazine 278 (December 2000)
A Dwarven Lexicon (p44)
Other than giants, all the races you ask about are described in the short passage I quoted above. This refers largely to the Forgotten Realms setting, as the previous paragraph mentions the dwarven god Moradin and how he created dwarves.
Giants
Volo's Guide to Monsters
Runes and Tale Carvings (p20)
This passage shows that either dwarves took the script from giants, or giants took it from dwarves, but which version is true depends on who you ask. This is also Forgotten Realms lore, due to the source.
As for why one would learn it from the other, it is likely due to the fact that (according to the 4e PHB and/or MM) giants had enslaved the dwarves in the past, and so the two races interacted closely and influenced each other.