National prestige.

[Raygereio]

people keep saying it's "hardcoded" - whatever that means

Hard coded means that data is inputed directly into the source code of a program, instead of the program obtaining that data from an external source.

For example, units in RomeTW are softcoded; the program checks external files like the EDU for the units' stats, the model files for how the units looks ingame, the EDB for where it can be recruited, etc. All those files can be easily (and legally) be modified.
Other stuff, like the AI, are hard coded; the stuff the program checks for that is inside the .exe and can't be modified (As far as I know, the only thing AI related that is softcoded are the formations that the AI uses. And you can mod that all you want, but give an idiot a stick, or a full hoplite panoply, it is still an idiot and will only hurt himself with it).

The only way to change hardcoded stuff is to obtain the source code of that .exe, change whatever you want and then recompile the program.
Getting the source code can be possible, but the EULA (that long text you usually see when you install a game and skip over) says you're not allowed to do that and SEGA is not going to give modders the code and permission to distribute it as that would be effectively handing out free copies or Rome Total War.

So long story short; hard coded means it's way more difficult and in some cases impossible to mod, but more importantly, you're not allowed to.
Where does that leave us with the diplomacy AI? Well, unless one of the developers posts all the information on the diplomacy mechanics, we can only guess and extrapolate from what we observe.

[Subotan]

Hmm. Interesting concepts. I'll bear that in mind when I start EBII.

SEGA is not going to give modders the code and permission to distribute it as that would be effectively handing out free copies or Rome Total War.

It's not like they make any money off it anymore. Besides, if I wanted a "free" copy of R:TW I know of various places where I could obtain it.

[Ludens]

So long story short; hard coded means it's way more difficult and in some cases impossible to mod, but more importantly, you're not allowed to.
Where does that leave us with the diplomacy AI? Well, unless one of the developers posts all the information on the diplomacy mechanics, we can only guess and extrapolate from what we observe.

Broadly correct, although hard-coded simply means it cannot be modded. It can be recoded if you have the source code, but that falls under programming, not modding. Very few companies give out the source code, and when they do it's generally for old releases.

For clarity: the source code is not some kind of password, but actually the program itself written in "programming language". Based on the source code, a compiler-program creates the .exe and assorted files.

It's not like they make any money off it anymore.

They do actually: they still get money for the sales of "classic games" editions, and so on. Also, parts of the R:TW and M2:TW code will have made their way into E:TW. Releasing the source code would be giving away their programming tricks to their competitors.

[Raygereio]

Broadly correct, although hard-coded simply means it cannot be modded.

I used the wrong word; I meant modify. That's what you get for not proofreading your posts carefully...

[Hax]

So long story short; hard coded means it's way more difficult and in some cases impossible to mod, but more importantly, you're not allowed to.

You are allowed to, if you have the source code and if you have received it in a legal way. Distributing is illegal. You might mod it for own purposes, yes, but once you start to give away .exe files you're doing illegal things.

[Kevin]

Nah, in my Romani campaign, I never break alliances but I still get that "An Alliance would not benefit us at this time." or the long block of text about how my faction is untrustworthy

[Macilrille]

Kevin, same here.

As for

It's not like they make any money off it anymore

.

I just bought another copy of BG The Entire Saga, so do not underestimate old games.

[Blxz]

Nah, in my Romani campaign, I never break alliances but I still get that "An Alliance would not benefit us at this time." or the long block of text about how my faction is untrustworthy

I know in M2TW they have the "Trustworthy" and other ratings when you do diplomacy. Its formed from stuff like breaking alliances etc. But also from exterminating cities and executing prisoners. I guess the same thing is in RTW except they don't tell you what your rating is. So if you annihilate all the cities when you take them, then that might be the reason you are untrustworthy, even if your alliance backstab record is clear.

But then again, who really knows whats going on with the AI? It could be rolling dice behind the scenes for all we know, I find that easier to explain how the AI decides what to do each turn.