@Philweasel  
You’ll be surprised how many of those with spare money never bother to do even sandwich once in their life.  
(And by “too many” I mean any non-zero number)

For all her fanciness, she’s still a single lady living alone. Either she spends a fortune on takeaways, eats nothing but ready meals, or she learns at least some basic level cooking.

@Ciaran 
Yeah she cooked for Sweetie B’s party too. I think. She never cooks for Spike, though. All she does is kiss him. Poor Spike.

@hironakamura 
We’ve seen Rarity cook just fine, until her little sister “helps”.
 

Cooking? Well I think rarity could cook, but she normally doesn’t.
 
I’ve also not seen spike eat much beyond hayfries, gemstones, and some cookies/pastires provided by the cakes.

@hironakamura 
Spike teaching Rarity how to cook… someone should draw it!

Which episode is this referencing? If its the Diamond Dogs one, then its correct. Rarity didn’t cook him dinner, she gave him a blue gem. Besides, Rarity doesn’t look like the type that knows how to cook.

Actually, Spike was too moved to even eat the gem Rarity gave him in “A Dog and Pony Show” after digging up a wagon load of them for her. Kid really needs to stand up for the rights of the working dragon.

Is this for real or is someone making a joke?I never play this game.

Give Rarity a break, you think she might be tired? Especially after all the crap her “friends” put her through.
 
Do something when you’re tired,  
it turns out like crap!
 
Followed by more sad Rarebear,  
and more icecream,  
and more tummy aches.
 
Oooh, boy… 
Let me simmer…

@Mojo  
You can cook diamonds (+1500 °C) to get delicious graphite.

You don’t need to cook gems you know.

Well she does let him keep one or two gems…

While yes, amusingly brutal and all that… Technically that’s just one specific scenario that’s false, it doesn’t actually mean she did nothing for him. 
Like, if I have a friend do me some generic favor worth, say, 50$-ish of labour, and I thank them by giving them a 60$ discount on some generic service I do (clothesmaking, perhaps, since I’m Rarity in this scenario) 
And let’s disregard whether that’s excessive payment for a favor or not. 
The statement ‘As thanks for their help, I made them dinner’ is false, then. 
As is ‘As thanks for their help, I promised them a favor in the future’, or any other reward that isn’t a 60$ discount on the generic service.
 
Without further info, for all we know she took him out on a date as thanks, or she does cook food for him, it’s just more of a lunch.

That’s pretty brutal.