It’s because too many couples come with baggage. Often one is serious about farming while the other isn’t. Some have relationship problems they think will be worked out by spending a week or a month in the country. The disparity in the work ethic between the two individuals can be severe — if one is a hard worker and worth their evening meals and the other isn’t, it creates an incredibly awkward situation. Same if one of the two gets ill or injured; one will usually want to hang back to take care of the other. And the list goes on. This, of course, doesn’t apply to all couples. But it applies to enough of them to not be worth the risk.
For internships/apprenticeships, a lot of farmers find it flat out insulting for folks to apply as a couple — because at what other job in the world do you apply in bulk?
The farmer could spend more time screening intern/WWOOF prospects and setting up extra requirements for couples… but the (profitable) farmer would never do that in favor of spending more time in the field or working on her business. That’s why I made the distinction between serious farmers and, say, a Trustifarian running a berry patch in Hawaii who has other bank accounts for keeping the lights on. Few serious farmers — those who rely wholly or most on the farm’s income — host WWOOFers; by the time they get proficient enough to stop breaking things and making mistakes, they leave.
The couple would probably be better off interning at two different farms anyway — if there’s two people, it seems silly for both to learn the same information.