Two good reasons I can think of :
1) Argicultural fruit is much higher in fructose, which is linked to some very serious health issues like fatty liver.
2) The so called "anti-oxidants" in some plants AKA polyphenols, which demonstrate no anti-oxidant properties in vivo, are 95% eliminated from the body almost instantly like toxins, have been shown to be cancer promoting in large doses. Low doses=cancer prevention, high doses=cancer causing.
They probably act via hormesis, rather than as anti-oxidants, as there is no evidence that they are anti-oxidants, but there is evidence that they are genotoxic. They certainly are nothing like uric acid, q10 or melatonin, which are real human native anti-oxidants.
Black tea for example can cause leukemia in infants (via polyphenols).
Theres no doubt that northern african hunter gatheres ate fruit. But it had less fructose, and they didnt have the confused impression that the more you ate the better your health.
Eating moderate amounts of low fuctose fruit, better emulates paleo man, rather than eating large food pyramid or fruitarian amounts of very high fructose fruit. Some level of fruit/vegetable polyphenols, phytochemicals and vitamins are clearly health promoting. But fructose isnt, in excess healthy. Theres a rate limiting factor on how much fructose you can digest at once, and it puts a heavy load on the liver, because unlike glucose, or starch, which can be digested and absorbed into the blood, fructose must be metabolised in the liver. Hence why an excess causes problems.
Just google "high fructose corn syrup", if your curious about why fructose in excess is a bad guy.
TLDR: the liver doesnt like loads of fructose, and excess polyphenols may promote rather than reduce cancer.