Future-proofing (up until MToF), and just being explicit for clarity
Thomas Markov's answer shows that there is one monster in Mordenkainen's Tome of Foes that can target more than one creature; the Deathlock Mastermind. However, before that, the only reason for explicitly stating what is always true for each attack in Monster Manual and Volo's Guide to Monsters must have been for future-proofing, in case one day they bring out a new monster, like they eventually did in MToF.
It's also perhaps simply good practice for them to be explicit to remove any doubt that an attack was intended to target only one creature (an easy mistake for players or DMs to make when considering gargantuan creatures; I can easily imagine someone thinking "but this creature is massive, of course its tentacle attack could hit all of the party members at once" were it not explicitly stated).