"Les hommes sont des garçons."
There is, in fact, a reason. When you are not familar with a language it can be difficult to recognise all of the words, this task is made more difficult when you are hearing words which do not form logical sentences because your brain is telling you "it can't be that because that does not make sense" thus leading to more mistakes, which leaves people feeling cheated especially if it was their last heart.
I am not saying that the sentences all need to be perfect, but there is a valid reason for people to expect them to be as it would aid the learning process.
I thought this was "the men are boys," not "men are boys" which is slightly different (e.g. "nous sommes les filles" translated to "we are THE girls" in another question). I am missing something... Could someone explain why "les" is used if "the" isn't in the translation? Why not "des hommes sont des garcons"?
"Les hommes sont..." expresses a generality: in general, men are... In English, no article; in French, definite article.
"... des garçons" defines what these men are. Put it in singular and you get "... is a boy". In English "a/an" has no plural form; in French, un/une has a plural form: des.
Here is what is wrong with your proposal:
"THE men" does not fit, because in English, generalities are expressed without an article and in French with definite article "les": "men (in general) are ..." = "les hommes (en général) sont..."
"des garçons", in this sentence cannot translate waiters, because if we wanted to mean which their job is, we would say "garçons de café" (=waiters). If Duolingo shows that "waiter" can translate in "garçon", the reason is that if you are in a café and you want to call the waiter, you can say "Garçon !". In that case, it is an address to someone, like a title.
On top of it, the expression"garçon de café" has been replaced for years by "serveur" (feminine: "serveuse") to define the job. Nowadays, if you are in a café or a restaurant, you will not use "garçon !" to call the waiter (who happens to be a woman in many cases), you will call: "Monsieur !", "Mademoiselle !", "Madame !", or more simply: "S'il vous plaît !".
if the English were given first, with this sentence: "The men are waiters", you would translate by "Les hommes sont serveurs". The singular form of the latter sentence is "The man is a waiter" = "l'homme est serveur". Note that to express a profession, the English use the indefinite article "a/an " (which has no plural form), while the French don't use articles. "He is a doctor" = "Il est médecin" => "They are doctors" = "ils sont médecins".
To sum it up: "Les hommes sont des garçons" expresses a generality (re. LES), and defines those men as being boys (which is not a profession), with "des garçons/boys" is the plural of "un garçon/a boy".
--> "Men are (former) boys" = "Les hommes sont (d'anciens) garçons"
--> "Men are human beings" = "Les hommes sont des êtres humains".