I just can't believe how a single modder has solved the Siege problem so perfectly and TaleWorlds with 100 employees still hasn't managed it after 1 and a half years. What could be the reason?
To answer the OP's question...
We don't know. But can hazard a number of reasonable guesses:
1. Modders aren't concerned about knock-on effects that the changes they might make have on other aspects of the game. Devs are, so have to take a more holistic approach to changes.
2. For devs,
Efficient changes that might be necessary might be tied up in other aspects of the game that are either not complete, or unresolved.
3. For devs, It's possible that there may be planned changes to large chunks of code that might impact on siege behaviour, rendering solving individual issues in current sections of code a waste of time if larger rewrites are scheduled.
4. It may be considered lower priority than other issues, therefore down the to-do list.
5. There may have been some unforeseen issues with required fixes that have left the issue on the back-burner.
6. It may be tied up in other behavioural mechanics that are still unresolved
But the one thing we know for sure, is that there is a reason - because we don't know what that reason is, we can speculate. How we speculate depends on our personality. Some will joke about it, some will cry about it, some will be aghast as to why, some will rationalise. But all is just speculation.