DFA and Dean Foods were more or less running a monopoly together in many regions already (technically not a monopoly since there was more than one company involved, but for all practical purposes it was a monopoly).
But monopoly aside, it's not like there were companies that DFA outbid. Most other dairy companies don't want to expand their low margin fluid business and only offer it as part of a wider range or products.
So this is kind of like the union situation.
Unions had the options of accepting a few concessions or else risk losing their entire CBAs.
Here the DOJ has the option of allowing this sale to go through even though it gives dairy farmers fewer options to sell their milk or else the DOJ blocks the deal, plants close, and dairy farmers have fewer options to sell their milk.