Is that true? I know nothing about the dairy industry, but human wet nurses aren't constantly pregnant so they can do their job, as I understand it production's just a gradual response to the 'request'.
It appears you may be right to some extent, but a bit of reading indicates dairy farmers to to have their cows birth a calf every one year to 400 days.
Older practices may vary, haven’t looked in to that.
Milking cows need to be having calves regularly so they keep producing milk.
Not owning a bull doesn’t make that problem go away.