You can still buy alarm clocks too, even though your phone has one, just like it has a flashlight. Cheap alarm clocks are so cheap that only a slight benefit like always having it on your shelf is enough of a reason to buy one. (Expensive ones are decorations and not mainly bought in order to tell time.)
I still prefer a real watch – taking a peek at my wrist is easier than having to dig my phone out of my pocket, flipping open the lid of the case and turning it on. That's true both in summer (just need to lift my wrist) and winter, too (I might have to dig my watch out from underneath my jacket and gloves, but to take out my phone I'd have to take off my gloves, too, so still a more cumbersome procedure.)
Plus lock screen clocks rarely (never?) seem to come with a seconds display (even inside the full clock app I still need to flip a settings switch in order to turn the seconds display on) – while I don't necessarily need actual seconds accuracy that much, knowing whether it is xx:xx:05 or xx:xx:55 certainly does make a difference when I need to catch a train/tram/bus/… and am cutting it fine once again.
Agree alarm clocks are cheap, but there's a very good reason to have one. Single use devices remove another area of phone dependence. Switching from waking up to phone alarm to dedicated clock alarm has been a huge help for me. It allows me to charge my phone in another room and create at least one no-phone zone in the house.
Watches have gotten less popular though.