hashkafah philosophy - Why was an imperfect world instructed in capital punishment?

So, according to wikipedia (I know... but still it was the best source I could find)

Leading rabbis in Reform Judaism, Conservative Judaism, and Orthodox Judaism tend to hold that the death penalty is a correct and just punishment in theory, but they hold that it should not generally be used (or not used at all) in practice. In practice the application of such a punishment can only be carried out by humans whose system of justice is nearly perfect, a situation which has not existed for some time or never existed at all.

Why then did G-d instruct Jews in this manner? Do I have to interpret this as meaning that the old Jewish society is considered nearly perfect (which I have a hard time believing in the light of the mistakes that were made as described in the Torah)?

Answer

I'd like to answer along two dimensions, one about capital punishment and one more broad.

First, it is possible for the conditions to be met under which capital punishment can apply. Tractate Sanhedrin in the talmud discusses in great detail the relevant laws. We know that sentences of capital punishment were carried out in the past. They were rare, with Rabbi Eleazar famously saying in the talmud (Makkot 1:10) that a court that executes once in 70 years is bloodthirsty (others say once in seven years).

Because these laws can be applied -- not today, but in the past and maybe in the future -- the torah needs to give them and we need to learn them.

Second, the broader point: the torah sometimes gives us laws that we cannot carry out. According to tractate Sanhedrin (h/t user6591, DoubleAA), the laws of the stubborn and rebellious son and of the idolatrous city were never carried out. (However, some disagree.) So why were these laws given? I was taught that, according to those who say it never happened, it's so that we will have the merit of torah study in learning it. Even if we do not apply a law, God had some reason for wanting us to learn it -- some principle we can derive from it, perhaps. God, being perfect, gave us a perfect torah, even if we imperfect people cannot understand why in some cases.