Apple's phones since the X have poorly designed bodies for being far too slippery. Personally, I think metal and glass body phones are stupid. My phone is a tool, not jewelry. Just make them out of high quality plastic. I never had to use a case with my Nexus 5 or Sony Xperia Compacts.
That said, Apple could design the phones to not be slippery. Texture like knurling could be added to the metal or glass that would make for better hand grip but still faciliate sliding the phone out of the pocket. But then they'd lose revenue from lost case sales and damage repair. It's worse than neglect - they profit from poor design.
This - profit - BTW is also IMO why they increase screen sizes every. damn. year. If they kept the screen size the same, the phones would look even more the same than they already do, which would make them appear less new and worthy of an upgrade. The point isn't for it to be obvious. It's for it to be incremental, so you notice the screen seems a little nicer without it registering that it's much bigger.
I prefer iPhones overall to Android, but IMO they are well designed for Apple profit - poorly designed for ease of carry and secure grip.