Why do we assume that PHB rules apply to monsters?

The party is deep into my 5e-updated classic Greyhawk Giants series.

They are returning the body of a stone giant killed in the siege of Headwater to the stone giant’s clan. The Thane of the Stone Giants intends to hold Funerary Games in honor of his slain kinsman and invite the PC’s to participate. One of the games will be unarmed combat.

In calculating an unarmed strike from a stone giant, I understand that the damage from the strike will be 7 points (1+ the stone giant Str mod). For the attack roll, however, I was unsure of whether to give the stone giants their proficiency bonus.

This question about unarmed bugbears, this question about unarmed skeletons, and this question about unarmed Flameskulls (?!), all have answers which state that monsters get to add their proficiency bonus to unarmed attacks, with the last question containing an answer from luminary SevenSidedDie stating as a general principle that "creatures always have proficiency with their unarmed strike".

As justification for this position, these answers cite some variation of the Player’s Handbook, PHB errata, or D&D Beyond, quoting "You are proficient with your unarmed strikes." None of the comments question this justification, and some support it.

Perhaps it is because I came to 5e from earlier editions, but my working assumption is that monsters / NPC’s have stat blocks, not character levels, and don’t necessarily follow the rules in the PHB. I read "You are proficient" and intuitively feel like ‘you’ means ‘You PC’s’, not ‘All creatures’. I don’t see anything in the Monster Manual that says that monsters in general follow the rules of the PHB or are proficient in unarmed strikes; I do see sidebars noting that monsters are proficient with their "armor, weapons, and tools" and that grappling rules work differently for many monsters compared to PC’s.

Under the general principles of "there are no secret rules", and "abilities do what they say they do", where can I find a general statement saying that monsters follow the rules of the PHB, and that such "you" statements apply to them as well?

Related: Are monsters subject to the massive-damage instant-death rules?

Related: Is the telepathy rule in the Monster Manual only applicable to monster telepathy abilities?