Pinkie Pie: I mean, are you sure? I don't know if I have any material that can cheer up an entire grumpy town...
Rainbow Dash: Hey, it's not like the rest of us got anything better.
Applejack: Yeah, truth be told, we're all a bit too cynical and jaded. But you, you've got a knack for spreading cheer and goodwill. And whether or not every joke or musical number lands, the fact that you try and you care means the world sometimes. And I'd bet my bottom bit that everypony in Ponyville knows it! Nopony makes you smile like Pinkie Pie!
Pinkie Pie: Honestly, the fact that you all believe in me... has got me feeling a little glowy inside already.
Twilight Sparkle: Figure I can put on the Element now?
DM: Yeah, that's valid.
Applejack: Who's the nicest, funniest, rootinest-tootinest Element of Laughter who ever cracked a joke?
Pinkie Pie: I am?
Applejack: Darn straight. Now get out there and spread some cheer from here to Canterlot!
A couple of unique split-second screencaps here with Applejack putting on the Groucho glasses and shoving Pinkie onstage.
...Wait, why do ponies have glasses with a human nose on them? I know "because TV-show shorthand for 'comedy apparel'" but... why do they have glasses with human noses on them??
The clear answer is that the Mirror Portal from Equestria Girls is shockingly lacking in customs enforcement. That, or there's a pony out there who said "What's the goofiest nose I could possibly imagine? Just the worst sniffer what ever snuffed."
Whatever the answer, the MLP wiki found this same question notable enough that it's listed on their humans page. Every instance of Groucho Glasses is noted, which is as impressive as it is frightening.
Honestly, I think it goes deeper than the portal itself. The parallels between the Equestrian and human sides are too numerous and widespread to be easily explained simply by direct contact through a single portal for three days every thirty months. There's got to be a more fundamental bleeding of ideas going on even when the portal is closed, and I do believe that runs both ways.
Well, Pinkie Pie does have some sort of bizarre link with her human counterpart, which I choose to interpret as all the Pinkies in the multiverse being connected.
I think it is because deep in the far reaches of the past, both worlds were literally the same. Or for my personal head canon.. humans were Exiled from the world that would become Equestria. Why are so many 'human' things in the land of ponies? Because humans USED to be there. I mean Starswirl liked to YEET things into the human realm, so maybe years and years ago the Humans were Yeeted themselves.
I remember back in the heyday of the fandom, questions like this would come up from time to time. And the answer was always the same: Pinkie Pie, that's why.
I don't think that quite works because it's not like we made the horse masks based on nothing (though I definitely had the same thought initially). We have the myth of horses going back centuries.
Maybe at one time they did know humans, and the comedy apparel stuck. Like, inter-dimensional portals, humans were wiped out, or we acknowledge some of MLP previous gens existed. All reasonably horrific in their own way.
...In my own ponyfinder games, it was inter-dimensional portals to Forgotten Realms, and the ponyverse stole power from them. Fueled cutie marks and the various alicorns and godly beings gaining power, as well as shut down weather and normal seasons and things due to the influx of magic. And also gave some of the monsters at same time.
Horses aren't just myths. I saw one once - I even rode it, honest! This was long enough ago that portable phones didn't have cameras, which is why I didn't take a picture of it.
And yes, I'm sure it was a horse, not a mule or a donkey. Definitely not a cow or a sheep.
Same reason that rarity knows what a tank is? The two worlds were once one, but it turns out a war between human tech and pony magic ends in a split dimension that slowly becomes an odd parallel system?
I'll risk ruining the mood by giving the silly question a serious answer. In G1, there was significant contact between ponies and humans. G4 never shows a human, but it does make many references to past events that were depicted in G1, implying that parts of G1 can be considered soft canon. The reason humans are no longer around is never addressed, but the fact there are not currently any humans does not mean that there were never any humans.
Also, have we ever considered... why the glasses have arms that would go around human ears, but clearly don't go around pony ears? Like... glasses are held on by the arms. What keeps them on if they don't hook onto your ears or something else?
...Wait, why do ponies have glasses with a human nose on them? I know "because TV-show shorthand for 'comedy apparel'" but... why do they have glasses with human noses on them??