Because C++ is not a "productivity language", it's a "systems language".
You can only trust expert programmers to write good C++ code, because C++ makes it way too easy to write code that appears to work, but is actually corrupting memory, leaking memory, etc, etc...
When using a more modern language, like C#, Python, Lua, et al, you can implement more features in less time, using less code, and with less bugs. Many of these languages also have great support for dynamic programming -- so for example, if your game "crashes" on a line of bad code, you can fix that line of code while the game is still running, hit your "reload code" button, and the game continues on without crashing! Same goes for tweaking/iterating features -- you can tweak your pathfinding code while the game is running, and see the results of your changes immediately, without having to quit/recompile/reload your game.

The usual practice is to have your expert systems programmers write the low-level engine functionality in C/C++ (a systems language), and the game programmers write the game in Lua/Python/C#/etc (a productivity/"scripting" language).

So the pros are: less development time (== less money required to make the game).
And the cons are: more memory usage and slower performance in the game code (which is why the engine is still usually written by C++ experts).

In these contexts languages like Lua and Python are mostly used as scripting languages inside that C++ environment
The benefit of this is that scripts do not have to be compiled and are interpreted at runtime, this allows for on the fly editing of code without having to recompile every time you change something. This is a major advantage for large projects since compile times can become long.

The downside is of course that these scripting languages probably won't run as fast as your native code does, so time critical code would still need to be written in the native language.

EDIT:

Damn, ninja'd by Hodgman

I'd like to add, as a general rule don't mix languages unless for the popular combinations such as those that have already been brought up above. I've dabbled for 6 months in C++/CLI and converting array's of bytes trough another language just to get it's data was a very tedious process. I literally spent one month making an interface DLL that would be compiled in /clr. My other choice was creating a servery with some TCP message forwarding the results to the other application.

It was sort of hit on already, but I'm curious if using scripting language as a one-man team would be helpful. I use Java for what I'm doing now, but if I can make my life easier by dabbling in Python a bit, that would be great.

Any one-man dev teams here with experience in using a scripting language with their games?

Different languages have different properties, advantages and disadvantages, the goal is to use the right tool for the right job, if your game is rather small and you are most familiar with C++ and already have an engine up and running, don't try to bring in a second language, unless you need to or unless you are just aiming at a learning experience rather than a finished product.

Another combination that wasn't mentioned comes in when games use databases, you'll have to integrate SQL.

Lets say you are aiming at a learning experience and go back to the script purpose usage of light languages (C#, LUA, python, AS) combined with C++ for performance.
What you want with a script language is a way to change the stuff that is most likely to change by design quickly and if possible on the fly, without having to close the app and compile and so on.

Take collision detection for instance, finding out if two boxes are overlapping is a rather rigid algorithm, you'll write it once and as long as it works ok you'll probably won't touch it again except perhaps to optimize some of the math. What you do when the collision happens on the other hand, is a design decision, for instance, object Bullet collided with object Avatar, C++ will tell you that happened, then you go to script, lets suppose its LUA and call the function AvatarGotHit, there you will take out HP points from him, the game designer then wants to add a shield, that protects from damage, then he wants the shield to be useless if the avatar was jumping at that moment....

You get the idea, the most efficient use of a scripting language is to use it to interpret a certain event that can be highly circumstantial and decide what to do about it, the actual doing of that will usually be best done back on C++
It is also, as has been mentioned, very useful to the balancing process, you have the "Jump" function in C++, how high the player can jump would be a number to be read from the Scripting language, that way the game designer himself can take the player, jump, change the jump height and try again until he likes it without bothering a programmer once.