1) Science now allows us to avoid the need for ruminants via direct cultivation of vitamin B12 from bacteria, or synthesize it directly (probably more expensive)
2) Milk and eggs
1) Yes, modern science can help us get vitamin B12 artificially. But 2000 years ago it wasn't possible. So instead of saying "you guys need vitamin B12 to survive", God said "go eat these animals on my approved list".
2) Drinking milk but not eating the cows would be a waste. Eating eggs but not the hens would be a waste.