I don't think they need to say why. Because the new players will just learn it via their experience playing it.
I always get the feeling that many veteran players have this need to continually "protect the new players" by making sure that anything that could go wrong is sanded down and smoothed out so that those issues never happen. As though these new players are like toddlers and we have to keep them safe at all costs... lest they decide D&D "isn't for them" and they stop playing. Everything should be explained away so that these players "know what they are getting into", rather than just figuring it out for themselves.
But in truth... more often than not these "rough edges" that veteran players wanted sanded down ostensibly to "protect the new players" are really just burrs under their own saddles that they want removed for their own sake more than anything else. They don't like some particular set of rules so they come up with reasons why those rules don't "help new players" to be the reason why those rules should be taken out or changed. When in actuality... those veteran players are so far removed from what they actually do need or want as a new player that no one can take their claims on faith. They haven't been a new players for years, if not decades-- especially not new players who have experienced games of other types that have tropes or concepts adapted from or taken directly from RPGs-- so what they think is "necessary" to play the game as a new player is not in fact true.
Which means that these recommendations that game is giving that the Core Four classes are your typical party composition is not in fact "necessary". And heck depending on how a particular table of new players plays... might not even be true. They may play a game with 4 Barbarians and find the game plays fine just because of how their DM runs things. But that's not to say this information is completely useless to everybody nor is it actively a hindrance even if it isn't necessary or required to be there. Some people might find it useful, some won't. But that's okay. It's there to be used if some table wants to, but they don't have to if they don't want. Just like the game gives 1st level spell recommendations to starting characters and offer up starting equipment. And players won't know if they are useful until they try and use them. But a veteran player making a declaration that they absolutely won't or absolutely will is a bunch of hooey. They have no idea either.