1) There are multiple essential nutrients not in any vitamin/mineral supplement pill, or not in nearly enough quantity. These range from Calcium (only about 10% of RDA common in multivits due to its excessive bulk) to Essential Fatty Acids (none) to some obscure minerals to the various essential amino acids.
2) There is reason to believe that synthetic-origin vitamins are less useful to the body than biologic-origin ones. 99.999% of vits (other than "E", which mainly comes from wheat germ oil) in multivits are synthetic, FYI. The reason is that the specifications on what constitutes a vitamin when synthesized are too lax. For example, synthetic Bs commonly are sold as a 50/50 mixture of "right" and "left" handed molecules, called in Organic Chemistry a "racemic mixture". As the body only uses one "handed" type (enantiomer, or optical isomer", the other half of the vitamin you paid for is waste at best, if it doesn't go on to interfere with absorption/utilization of the "good" kind (which it may).
3) There are nutrients not yet identified, so cannot be added in pure form to any multivit pill. It is an axiom of industrial microbiology (which I have worked in) that adding a complex nutritional supplement (cane or beet molasses, organ meats such as liver, blood, milk, etc., depending on the organism) to a fermenter will speed up growth beyond any "pure" nutrient mixture that can be devised.
4) There are a few desirable compounds found in certain foods worth going out of one's way to add to one's diet where possible. Examples include flavonoids found in blueberries/purple (NOT green!) grapes, lycopenes found in highly processed tomato foods, and of course Omega-3 oils as found in deep coldwater fish (haddock, cod, herring, nonfarmed salmon, sardines, mackerel, swordfish, anchovies, etc.) and to a lesser extent in flax products. Again, these will rarely if ever be in multivits at all, and never in sufficient quantities to get maximum benefit.
I do use multivits on LD hikes, but am keenly aware of their lacks.