We are probably not eager to spend time writing tests right now. We don't want to lose the forward momentum we are feeling. Just as we believe we are making some real progress, stopping to write tests feels like make-work. It takes away from the joy of making yet another series of improvements to the awful codebase that we have been suffering under for so long.
The resistance to writing tests is understandable. I myself was only a slow convert to automated testing. If one is not used to it, the act of writing tests feels alien, unfamiliar, challenging, and unproductive. It is very easy to say, I can see the code is working, because the application is working.
And yet if we do not write tests, we are condemning ourselves to continued rounds of suffering later. We are enabling a particular kind of awfulness in our legacy application: that feeling of dread we feel when we change one part of the application, not knowing what other parts of the application are going to...