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On a more general note, a human eating a vegetable is a predatorial relationship, and like any prey, the vegetable will evolve defenses against such predation. Obviously the vegetable can't run away, so it produces compounds that are harmful against its human predators. Artificial selection through farming may reduce the harmful compounds, but constitutes a small fraction of their relevant evolutionary history.

On the other hand, we have the opposite relationship with fruits. When our ancestors ate fruits, they played a pivotal role in facilitating the plant's reproductive fitness by dispersing the seeds, at times with the excellent complimentary fertilizer that is human excrement. In this case, the plant has an evolutionary incentive to produce more nutritious fruits to encourage such human consumption.

There are a lot of "vegetables" that are actually botanical fruits such cucumbers, zucchini, tomato, etc. (note they all contain seeds). They are much better alternatives than true vegetables (spinach/kale) for juicing if you are trying to avoid sugar or somehow don't like sweet juice.

It also doesn't sound right to me to refer to liquified vegetables as "juice", when I've long thought the term refers to the liquid produced from squeezing a fruit (not to mention liquified spinach/kale sounds a bit nasty)