Because, Roblox doesn’t have to deal with App Store Rates on the Computer and Web versions of Roblox.
The entire idea is to make people desire buying Robux on the Website or on their Computers,
so that they get more Robux for the same amount of money they usually spend.
But, Roblox doesn’t have to give any cut of that money to Apple or Google if it’s not on the IOS/Android Roblox Apps.
Ta-da! 