this was something that i put much thought into.
since i rewrote how siege units are in my mod.
as f1rpo stated , ai value is a start.
i found that i changed the dll in several places to get proper ai usage of siege.
this isnt an easy task and require much testing.
if you dont want to complicate stuff,
give the siege units some more valuable stuff for the ai -> ai weights as said, more strength / bonuses in battle, movements.
if you fear that giving siege more strength, you could negate this by giving extra defense to all other units, lets say unitcombat vs siege 10%.
in that way the ai will think siege are stronger and build more, but infact all units have more defense vs them.
also - combat limit lowers the build count of siege units - think about that as well.
could also change its unitai - attack, attack city, counter and such.
VIP , Civ4Reimagined, RI and my own mod, doto, did some major changes to how siege is working.