Because TransferWise is trying to solve a problem and keep it solved. Meaning - lowering fees as the userbase grows, only as much so it can sustain operations, not losing focus on what the problem is, sustaining a good company culture and not burning people out. At the end of the day this company needs to last, otherwise banks start raising fees again. This is not some joke startup that is aimlessly looking for a way to make money, unlike some competitors.