It is mostly because they are unofficially counterparts. Since Miltank is only 100% female. And Tauros is only 100% male.
Tauros and Miltank have other things in common. They have the same Type (Normal), same Base Stats (490) since all the gender counterparts have the same Base Stat, I think Tauros and Miltank are gender counterparts, since not that much Pokémon have 490 as a Base Stat.
Same Egg Group (Field, okay really much Pokémon have the Egg Group Field but whatever) and like I said with the Base Stats, all the gender counterparts have the same Base Stats, that means same goes for the Egg Groups.
Something else that they have in common is: Tauros is a bull, while Miltank is a cow. Bulls and cows are similar to each other. Cows are female, and bulls are male.
And something that is some random stuff and doesn't prove anything is: same Egg Cycle (20 base 5,120 steps),
same Growth Rate, same Catch Rate (45%), both have three Abilities.
So why are they in the same 'Dex page?
Mostly because they are unofficially counterparts. Maybe Let's Go Pikachu/Eevee will give us a pre-evolution for Tauros and Miltank.
Source: https://pokemondb.net/pokedex/miltank , https://pokemondb.net/pokedex/tauros , my head :P