The reason is software. Writing good quality software that works on both touch screen and mouse+keyboard is expensive, time-consuming, and tricky to get right. Not only do you need to write apps that work in both mediums, but then you have to write the bridge that transitions between the two modes. Do it wrong and you get Windows 8.

So why bother? Microsoft just kinda allowed touchscreens in laptops, but Windows is still very much a mouse+keyboard-first OS, and touch comes 2nd. For Apple so far, it’s been cheaper (and more profitable) to sell separate devices, write separate apps, and use Continuity/iCloud/Handoff/AirDrop to transfer between devices.