Additively, you need to know how your 1 addition interacts with all n elements of the existing system, so there are n things you need to understand. Subtractiv

Additively, you need to know how your 1 addition interacts with all n elements of the existing system, so there are n things you need to understand. Subtractive, you need to do that same analysis for every part of the system you could potentially remove, so you're doing n operations n times each. The subtractive analysis only becomes equivalent to the additive one if you know a priori which specific element you ought to remove. Placing a jenga block, you can choose to put it in a location that is relatively easy to analyze, for example the middle of the next layer up. Removing a jenga block, you first need to decide on which block to remove, and then you have to deal with the situation it actually happens to be in. Either action could knock the tower over, but the odds of you getting something wrong go up dramatically during the removal phase.