Answer:
The easiest argument is the next one:
Suppose phytochemicals are like money. If I put you in a phone cabin with a lot of money and give you 30 sec to be there collecting as much money as you can, at the end you will collect not all the money and a lot of money will be lost. The same happens whit phytochemicals in supplements there are so many that your body can't use them all.
And other thing is that phytochemicals in supplements comes so concentrated that your body will be overstimulated to take this phytochemicals and that changes your normal digestion, in the time it may looks good...but if you don't take supplements anymore, your body will feel the lost and the physical results would be awull.