These are often long term projects; consultancy firms, especially big ones, are very good at delivering over time and over budget and still remaining there 10+ years. They are trained to do this and do it well. Maybe it depends on the region but it's very normal in Europe anyway for the Cap Gemini's etc to do this.

Sure they have an unending stream of work, but you rather still do far less work for far more money or rather; be able to bill far more hours for the same end result is preferable for profits. The art is then to still have a ravingly enthusiastic client at the end of the project. It's an art. Or a scam. As a techie I find it the latter but I admire how they keep the client happy, time and time again.

although the problem is there's no incentive to perform when delivering late and over budget rarely has any consequences (on future work streams)

although the problem is there's no incentive to perform when delivering late and over budget rarely has any consequences