If you can't rely on your customers always having an earthed outlet to hand (as you can't in countries that permit unearthed sockets, or often have dodgy electrics), you have to design it to work without the earth line in case a customer uses that socket.
Adding an earth conductor would require a thicker, less flexible 3-wire cable, a bigger plug (in the countries that have the option of a 2-pronger) and a three-pin power connector if it's not a captive cable, which is bigger and heavier. To ground that Y capacitor node means taking the earth conductor from the socket to near the SMPS transformer, and you have to maintain strict clearance and creepage distances between that conductor and the live/neutral regions at all points. Modern power supplies are very compact, so this is actually more of an ask than you might expect.
As the device is already designed to meet or exceed safety requirements without the earth wire, all it does is avoid the tingle and add weight, volume, design effort and component cost. As mass market devices produced by the hundreds of millions, this is not deemed a good tradeoff.
You can avoid this on your own, however, by grounding the case yourself, by connecting any exposed metal to a local earth point. Remember that Macs are anodised, so the surface isn't really conductive, (which actually amplifies the effect at the point of contact as a thin insulator is also a capacitor!).
I see! Apple had the option to supply a fat three-prong plug here, which wouldn't fit in any unearthed socket, except for fake ones (which do exist but are generally older extension cords over here). But as you say, they likely didn't think this was worth the trouble + customer inconvenience if they couldn't rely on earthing elsewhere.
Adding an earth conductor would require a thicker, less flexible 3-wire cable, a bigger plug (in the countries that have the option of a 2-pronger) and a three-pin power connector if it's not a captive cable, which is bigger and heavier. To ground that Y capacitor node means taking the earth conductor from the socket to near the SMPS transformer, and you have to maintain strict clearance and creepage distances between that conductor and the live/neutral regions at all points. Modern power supplies are very compact, so this is actually more of an ask than you might expect.
As the device is already designed to meet or exceed safety requirements without the earth wire, all it does is avoid the tingle and add weight, volume, design effort and component cost. As mass market devices produced by the hundreds of millions, this is not deemed a good tradeoff.
You can avoid this on your own, however, by grounding the case yourself, by connecting any exposed metal to a local earth point. Remember that Macs are anodised, so the surface isn't really conductive, (which actually amplifies the effect at the point of contact as a thin insulator is also a capacitor!).