There is nothing “random” about value, but it is certainly personal. It is value that motivates people to take specific actions, thus giving rise to the economic phenomena that we study—such as prices, resource allocations, and economic growth. There are perhaps reasons (or causes) for why we value one thing but not another and why our valuations change over time, but those are issues in the realm of psychology. Economics does not and needs not deal with why people value something (or not). What matters is that they do and that they take actions motivated by that value, which have consequences in the physical and social world.
There are many more problems with mainstream economics, but core to them is the treatment of the economy as objective and therefore measurable, whereas any economic phenomenon is necessarily based on value subjectivity. This does not make economics less reliable as a field of scholarship. On the contrary, pretending (or “simplifying”) that people make objective choices between objective alternatives is the cause of many errors by economists.
To study mere choice excludes much of the creative, value-motivated, temporal process of action and therefore the cause of social and economic phenomena. While the economy can be studied from the perspective of objective, measurable data, the problem is that no actor within the economy—whose actions in effect make up the economy—acts based on objective parameters. Value is at the core of the individual action—what motivates it—and, therefore, value is also what causes and gives rise to all economic phenomena. The fact that many economic phenomena are objective, or at least intersubjective, is in fact irrelevant to the study of economics because economic phenomena cannot be understood without recognizing and consistently applying value subjectivity.