Because it creates a secret layer of complexity in making a build. Now you make micro adjustments before entering any activity. As the number of activities increases (which it is about to), this burden of optional play becomes a tedious part of the process. Build guides also suffer from the added complexity of the node recommendations for each of those activities.
There are things to consider that might not be obvious. This requires us to save new information in a new format. Now the saving and loading process has to be adjusted. It makes adjustments to that more complex in the future too. The UI needs to be created for it, that UI needs localization done for it. The game guide needs updating to explain it.
Each department that touches the system has all of their other projects affected for time too. I know it seems like a super easy, obvious win but a lot goes into things like this that are very hidden. We have to weigh the value of it against all those dev hours.
I'm not saying that it's all bad, it does good stuff too, it's just not so cut and dry.