The lich in the original 5e Monster Manual had the Paralyzing Touch spell attack. However, it’s not a spell available to the PCs. Since you care about verisimilitude so much, whenever the first time you came across that obvious error, did you immediately homebrew a Paralyzing Touch spell for your players to learn once they ran into the lich.
What about the Necromancer from Volo’s Withering Touch melee spell attack? Or the Dusk Hag’s Nightmare Touch? Why aren’t there rules for turning yourself into a lich or creating a Beholder Zombie? Hell, why are spellcasting monsters held to this standard, but martial ones aren’t? Why can’t a Fighter learn to use the Knight monster’s Leadership action? Or the Tribal Warrior’s Pack Tactics? Or the Bandit Captain’s multiattack? Why does the breath weapon of a Drakewarden Ranger deal less damage than a typical dragon breath? Why are the Assassin’s attacks automatically and permanently covered in poison that deals 7d6 poison damage if you fail a Con save? Isn’t it unfair to the Rogue that they can’t learn to do that? Why do monsters get Legendary and Lair actions but the PCs don’t? It’s not consistent or verisimilitudious. That is what makes good game design according to you, right? Everything has to be internally consistent and follow the same rules?
Monsters and player characters do not follow the same rules in 5e and never have. I’m not sure if you haven’t noticed that before, but you’re not holding the original 5e books to the same standard you’re holding the newer ones to. And you’ve mentioned several times that you haven’t read the newer books. You won’t even watch these videos about the newer books to hear their explanations for the changes and their design philosophy, yet you spend so much time complaining about them in these threads.
Yes, 5e always had this sort of spell attack that the PCs couldn’t learn. Yes, post-MotM 5e has more spell attacks for NPC spellcasters that the PCs can’t learn than it did before. No, Monsters and PCs do not use the same rules. No, internal consistency doesn’t make good monster design. I’m glad that I don’t have to ask my players for the Player’s Handbook to look up a spell in the middle of combat as much as I used to using the older version of monster spellcasting.