There are a thousand low level details in the answer to this question; I'll try to cover some of the basics.

A high end vitamin tries very hard to package nutrients from actual food sources (i.e. extracts and concentrations of something you would really eat) instead of just cramming in raw materials. This tends to mean that the bioavailability (i.e. how much you actually absorb instead of just pass through) is much better. For example, sourcing calcium from ground up oyster shells provides plenty of the mineral, but the absorption rate is terrible. Much research goes into finding the best bioavailable forms.

In general, the ingredients are significantly better quality. Packaging B12 as cyanocobalamin may cost 1/10th as much as hydroxlcobalamin, but isn't nearly as good for us.

The ingredients are plentiful. The FDA's recommended daily allowance numbers are WAY off in most cases. For example, my multivitamin packages over 3000% of vitamin C, and over 8000% of B1.

Finally, a high end multivitamin includes a huge range of stuff beyond just the basics. For example, look at the ingredients list for the LEF Mix Tabs. It's 4 pages long in my browser. If you're curious, I suggest Google or Google Scholar around for the specific chemical forms of all the vitamins and minerals mentioned.

Nope. It's just that Big Supplement wants a way to make more money off the gullible :P We really don't need that many supplements. Decent food and no absorption issues is all it takes.