Synthetic multivitamins in no way compensate for a proper and nutrient dense diet. The nuances of how vitamins and minerals are absorbed is also key, just because you take a multivitamin, does not mean you’re actually absorbing anywhere near 100% of what is inside it. Not eating varied foods including plants means you are also missing important antioxidants and beneficial phenols and other compounds.
Lol I may have done this for like a year as an idiot. Turns out malnourishment is a real thing. Microbiome went down the drain as well. No fiber no Phytochems literally no information from the food to the microbiome , no dice.