Google isn't in the business of selling phones.... yes, they sell them, but they don't rely on them for business. And Android does have external storage support... in fact they've spent quite a bit of work in recent versions to completely overhaul how Android handles SD cards, specifically to reach a difficult balance between functionality and security. If Google had some deep seeded desire to end SD cards, they could have just dropped support in the OS and be done with it. But they didn't.
Adding card support to a phone is a pain in the butt, and that's ignoring the fact that if you use one, it is the single most unreliable piece of hardware in that phone.... but a significant margin. Quality control in the industry is a joke and the market is flooded with absolute pieces of trash SD cards...slow, buggy, prone to corruption, etc.
So Google takes the stance of letting people who want to deal with that headache do so, but they take a pass in putting it into their own phones.