Simple: time is a resource, it is a cost. When one bids for a project, time is also taken into account when selecting a vendor. So, just as one tries to offer the best price and experience, the same goes for time. One knows that the project will most likely take longer but if that realistic timeframe is put into the proposal, there is risk in losing the contract. It is a safer bet to later ask for reasonable time extensions than not getting the project at all.