There are two main ways to force the AI to pit. I have experimented many times over the years with rF and GSC - I sometimes seem to spend more time trying to create my perfect offline season than actually racing it LOL..
The first way is through fuel. You can use the fuel multiplier to increase their consumption. This method works very well, and is a reasonably 'realistic' method, in so much as they start with a full tank, and gradually go faster till they need to pit and refuel again.
But this is no good if the player cannot also choose some refuelling options to put them on a similar strategy. Pit strategy is part of F1, even if it was a bit random in some years.
Or (still on the fuel method) you can manually edit the fuel the AI use. With access to the fuel data, you can do several things - change the initial amount of fuel in the tank, or even change the size of the tank - this works well because we know that the AI will pit when they are running out of fuel, so I like to edit the tank size to control their pitting.
The hdv file that contains this info is not visible in GSC, so these changes can not be made this way, and again the player must also be able to choose a strategy, hence I was asking for fuel options to be made available.
(it is still possible to make changes through upgrades but any of this editing does need patience & some knowledge, hence my reason for asking for some convenient fuel options to be reinstated).
The only trouble with these methods, as Reiza point out, is that in F1 eras where there was no refuelling, the full weight of fuel was important as it made a bigger difference at the start of a race and gradually affected speed, handling & lap times through the race.
The second method is much easier and guaranteed to work, but is maybe not as attractive in a simulation because it is already written in stone - in other words, you can tell each AI driver how many times to pit and on what lap(s).
To do this, see my previous post:
http://www.racedepartment.com/forum/threads/mandatory-pit-stops.78173/
But again, the player must also have the ability to choose some pit strategy, or the AI will still be behaving differently.
So your assumption is right, the AI pitting is certainly a bit flawed, but it CAN be forced to work using fuel to control when they pit.
I hope that is of some help.