Diets with vegan restrictions consistently perform better than nonvegan diets in studies. Not all of those studies account for UPF consumption among other things, in which case vegans might just perform better because they are a more health conscious demo, but there are those that do account for that as well as interventional studies where participants do not choose what they eat.

Consumption of animal products is associated with some of the most frequent causes of death such as cardiovascular disease and cancer. One explanation of this are SFAs which have been heavily researched on their own, and are known to cause health issues even from plant sources like coconut and palm oil, but are found in most animal products whereas in most plant foods they are found only in small quantities. In addition you have animal proteins which have been shown to trigger the production of certain health negative hormones, likely do to their amino acid composition or just their quantity of protein in general, tbh I have not looked into the nuances of a excessive protein omni diet vs one with no animal products. You also just have the fact that many animal products aren't as (micro)nutritionally dense compared to many whole plant foods, even high calorie ones like pulses.

I'd be curious though as to know why omnis, especially scientifically minded ones, don't default to eating in line with vegan ethics. Since the vegan argument is an ethical one it should be on those who think we need to eat animal products to show why that is the case, animal products should have to be extremely necessary for our own survival to justify the harm we do to animals.