Diet is quite more complicated that just taking a multivitamin, unfortunately. Some nutrients compete for absorption (Zn and Ca, for example) and shouldn’t be taken together. There’s also debate about the need for phytonutrients that also come along with plants. The data is mixed enough that the US Preventative Services Task Force isn’t yet willing to endorse supplements as a means of reducing cardiovascular or cancer risk.

My personal opinion is that the human body may be too complex to say we fully understand a seemingly straightforward solution like just taking a multivitamin. We evolved over an awfully long time before industrialized agriculture and supplements. I’m not trying to demonize them because they have solved the number 1 concern humans had for the last 10,000 years of not getting enough calories, but I think it’s wise to temper the hubris of thinking we understand the human body well enough to expect a simple fix from a pill.