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.
Things go over time & budget for lots of reasons. The biggest one is that the lowest bid is likely to win so the incentive is to under-estimate. But it's rarely because the contractor is intentionally creating more work.
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.