Why does the Singapore's government have a monopoly on land, thus keeping prices high?

In Singapore, a good and effective housing and land policy play a very important role. A major portion of the revenue of the government comes from the ownership and sale of the land. Due to this, the government has a monopoly over land. Private individuals cannot influence the price of land in Singapore. Another reason for owning land monopoly by the government is that when companies and businesses buy land at high costs, they cannot exit from the business easily because if they do so they will have to incur very high sunk costs.