Because Bethesda have the skills, opportunity and tools to create things modders can't do, or would have to spend a lot of time doing.
I daresay Hearthfire will add numerous things that existing mods don't.
O rly?

http://skyrim.nexusmods.com/mods/9782/
Bethesda employees know the engine better than the modders, having more experience on it, and have access to the source code, meaning that they can hard code something which cannot be obtained via clever hacks or script alone.
On the other side...
While I don't doubt their creativity, their biggest limitation is that they work for a structured company with certain goals and schedules.
Last, Bethesda developers are hundred of people versus hundred thousands modders, some of which are professional coders and artists who use their talent not only in their profession. I think quite a few PC users will end up buying Hearthfire because its "canon" material even they could get free (and equally valid) alternatives.