Or they don't want to, at least not at this point. Owning an oven is pretty expensive, more so if you can't keep it baking things on a regular basis. Also, you continually need to keep upgrading your oven, adding to the cost. The cake recipes are also pretty complex, similar to building new ovens. However, Apple isn't doing it from scratch. They've got someone else's recipe that they can play around with and add their own ingredients to which makes the process similar.
Apple probably has enough device volume that they could just buy their own small fab at this point. The biggest problem is that they'd need to keep upgrading it and that can get expensive in hurry, which is probably why Apple is content using third parties to get their chips made. And for what it's worth, making a modern chip is pretty damned difficult. Just ask AMD and any other company that builds CPUs or SOCs. ARM makes it easy by providing a basic working design, but if you want to tweak it, it still requires a team of people that know what they're doing.