The answer lies in your question and I believe this is so in Magento and as well in lot of other e-commerce and accounting software cause of dependencies the "order document" has.
In commerce and accounting usually editing is not a way to do things and in order to achieve editing effect credit or debit balances on specific accounts are altered with adding new rows that either add or subtract values. It's not a bug and it's a feature as accounting means calculating the result based on actions not the count of the actions that are needed to achieve some result. Each edit is a action that needs to be stored in order to provide the possibility to calculate the balance, archive it and calculate or present it over and over again.
So in Magento the order in whole is not editable as it is not needed although it seems weird and unnecessary at first to almost all merchants don't get it. The fact that it makes harder to maintain your e-store is just a side-effect of doing things properly.
If you think further then even with altering all the order siblings (addresses, items, shipping and payment methods, applying the discounts) the same issues will happen if totals change due that. You still can't capture payments if you don't have the cc data , refund etc cause it needs a action from client. In e-commerce you only have the client attention at the time when he confirms the payment and everything that happens after that needs more attention from both partys
So even if in IRL at the store you are making some kind of ordering the same process of adding and subtracting actions are taken (by cash register or in our minds) to finalise your order and if you need to change after payment process the order is discarded and new one is created to record your new actions (payments, discounts, bonuses, items etc). Only thing that makes this possible in IRL more seamlessly is that client is usually there to confirm the changes and authorising the payments and all details and it still happens slightly faster in any brain cause relations are abstracted.