Selective breeding, of course. Modern vegetables are, in general, less bitter than in earlier times. Fruits are often sweeter---and easier to farm industrially and bring to market. Also, we live off less different kinds of fruit and vegetables than in earlier times. There used to be hundreds of different apples in widespread use for examples.
Not saying that it's necessarily bad. Just that pron's hypothetical argument isn't actually all that hypothetical.
That does not necessarily mean more sugar - it just means less bitter. (These axes are independent).
Selective breeding puts pressure on easy-to-grow-and-sell (shelf life, size, shape, resistance to pests) and only afterwards on flavor.
And another important (and often forgotten) point about modern frutis and vegetables is that they are mostly picked when green (when not all sugar has ripened yet), and then chemically ripened in a process that does not yield as much sugar as natural ripening.
I would like a reference before I believe that today's fruits and vegetable contain more sugar than those of 50 years ago.
And regarding "Getting all your vitamins from meat is ridiculously inefficient" -- you are trying to hack the unhackable, speaking in scientific terms about things we have little clear scientific understanding of. "Fruits and vegetables cut out the middle man; they use much less water and land to get the same nutrients into your body." -- Nope; you don't know that. Some people believe that, some don't.
For all we know, such careful nutritional hacking has little, if any, effect on our health. That's what we know. What people believe is a whole other matter. Most of the diet-talk we hear is just catechism of a new religion (and there's nothing inherently wrong with that, except that pseudoscience is a little annoying) and a whole lotta politics (in this case, it's fascinating to see liberals defending pseudoscience, while conservatives discard it, maybe because they believe the liberals who believe it's actual science). Science it ain't.
Why would modern fruit contain "more sugar"? More sugar than when? How much more sugar? Who miraculously put the extra sugar in the fruit? The fact is, even if we selectively bred fruit to be sweeter, we're not going to get it much sweeter than it already was. Nobody's going around injecting apples with extra fructose with a syringe.
Getting all your vitamins from meat is ridiculously inefficient; you can do it, but you're creating a tremendous ecological burden. All of that meat has to be raised, fed, watered, and pastured. Fruits and vegetables cut out the middle man; they use much less water and land to get the same nutrients into your body.
It's fine to say that 'magical thinking' is bad, but you're not doing any better. If anything you're ignoring facts based science in favour of 'what some people say'.