None of the DIY-Bratwurst recipes I know over here (Germany) contains milk or cream (although in general we love milk and add it almost everywhere :-) ).
The general composition is always meat (lean + fatty to arrive at 20 - 30 % fat), spices (salt, pepper, herbs, possibly garlic). Homemade Bratwurst usually avoids adding water/ice, and also avoids starchy components - they are mostly seen as cheap components to dilute the Bratwurst in commercial settings.
OTOH, a friend with milk allergy always asks specifically when buying ready-made Bratwurst to be on the safe side, and told me they sometimes contain milk/cream.
The Austrian Codex Alimentarius lists milk as possible ingredient only with very few sausage types. (And in general is more concerned with limiting the amount of water and starch in commercial Bratwurst)
The predominant meat for Bratwurst over here is pork, which is rather fat compared to other meats (also Speck is often added to Bratwurst that uses lean non-pork meats such as game).
If you have looked for pork-free Bratwurst recipes, the required amount of fat needs to come from somewhere and that may be the reason for e.g. adding cream as an easily availabe non-pork source of animal fat (maybe try goose fat?).