Mr. Li and Ms. Zhang are husband and wife, and together they conceive a son named Xiao Li. One day in 2015, Mr. Li, Ms. Zhang and Xiao Li signed a "Letter of Agreement", which was of great significance, marking Mr. Li and Ms. Zhang's decision to donate a house to their son Xiao Li free of charge. This "Agreement" is written in black and white, clearly indicating the willingness of Mr. Li and Ms. Zhang to donate the property in their names to Xiao Li. However, after the signing of the Agreement, Mr. Li and Ms. Zhang did not immediately transfer the ownership of the house to Xiao Li. They continue to live in this house, and perhaps in their opinion, the transfer of ownership is just a formality, and the family connection between them and Xiao Li will never change. However, an incident in 2017 shattered that calm. Xiao Li suddenly made a request one day, hoping that his parents would assist him in the transfer of the house and deliver the house to him. However, Mr. Li and Ms. Zhang categorically rejected their son's request on the grounds that Xiao Li had not fulfilled his maintenance obligations. They said that they were no longer willing to fulfill their gift obligations because Xiao Li had not fulfilled their maintenance obligations. This decision undoubtedly shocked and disappointed Xiao Li. 
After an argument to no avail, Xiao Li sued his parents, Mr. Li and Ms. Zhang, to the court, and he demanded that his parents fulfill their gift obligations and transfer the house to him. After the trial, the court rejected Xiao Li's claim. What is the specific reason?
According to the legal provisions of the gift contract, the donated property needs to go through the procedures for transferring the name and changing the name, and the property rights of the real estate will be transferred when the transfer registration procedures are completed. Before the transfer of the property, the donor can withdraw the gift. Since the gift involved in this case is a general gift, not an irrevocable gift under the law (such as a gift contract with the nature of social welfare and moral obligations such as disaster relief and poverty alleviation, or a notarized gift contract), the withdrawal of the donated real estate by Mr. Li and Ms. Zhang in this case is legal and valid.