Well there's also Riolu, Smoochum and Mime Jr.; though the former only got it in Gen 5 and the other two only have it as an egg move.
True, but I can sort of see those Baby Pokemon having it. Also the other Baby Pokemon don't have it and out of those I can see some which I think look like they could learn it:
Baby That Could: Tyrogue, Elekid, Magby, Bonsly, Munchlax, Mantyke
Baby That Couldn't: Cleffa, Igglybuff, Azurill, Wynaut, Budew, Chingling, Happiny
I have my reasons why and why not, but but Riolu, Smoochum, and Mime Jr. would have gone in the "Could" while Togepi and Pichu would have gone in the "Couldn't". Out of the Pokemon that looks like they would think up something nasty, you picked which don't?
Maybe Snorlax doesn't get Slack Off because it doesn't so much laze around as it does just sleep a lot? If Rest weren't a move that every Pokemon got, it would make a lot of sense for Snorlax to have Rest over Slack Off.
The description of Play Rough is the user plays with the target in a rough (aggressive) manner. It's hard to envision a huge tree deer like Xerneas playing with anything, and it's also considered a benevolent Pokemon so even if it did play, it probably wouldn't rough anybody up while it was at it.
Slakoth (and by extension I would think Slaking) also essentially sleeps all the time. FUN FACT: Apparently Slakoth's heart only beats once a minute... how is this thing even alive? Also it only eats three leaves a day... HOW IS THIS THING ALIVE?!
And who's to say that a Legendary can't play? Also you don't know how Xerneas would "play", sure its maybe not rolling around in the grass but I can see it kicking and stomping around (which would be painful with those blade feet). I also wouldn't imagine Absol or Skrelp getting that move either in your context but they do (sure its an Egg Move but it still counts). Actually Play Rough would be another move you'd think a lot of Baby Pokemon would know.