For Brevity. James Madison explained this fairly well.
If they had in general terms declared the Common law to be in force, they would have broken in upon the legal Code of every State in the most material points: they wd. have done more, they would have brought over from G.B. a thousand heterogeneous & antirepublican doctrines, and even the ecclesiastical Hierarchy itself, for that is a part of the Common law. If they had undertaken a discrimination, they must have formed a digest of laws, instead of a Constitution.
They had to leave the interpretation of the word "unreasonable" to some level of subjectivity because it would simply be too lengthy to articulate all such distinctions.
Well, you're suggesting that they used vague language because they believed it was impossible to be more specific. I am suggesting that they used vague language for an additional purpose - because they wanted to expand the power of future decision makers to determine what is reasonable or unreasonable under the different circumstances in which the future decision makers would be living. I am suggesting that they knew that circumstances would change and that future decision makers would make better decisions than the drafters could make about what would be reasonable or unreasonable under those changed circumstances.
Does that disturb you?