It's pretty simple physics that the tiny sensor in an iPhone(as good as it is) becomes unacceptably noisy at much lower sensitivity levels than a bit sensor. Even a crop sensor(~16x24mm) has a lot more area and consequently larger photosites better able to gather photons than a small smartphone camera. A full frame(24x36mm) sensor is typically even better, although there full frame sensors with about the same pixel density as crop sensors(i.e. the 45mp sensor in the Nikon D850, when used in DX Crop Mode, has about the same resolution of 20mp as the D500 crop sensor). Any of those are still better than an iPhone that packs 12mp onto a ~4x6mm sensor(rounded numbers).
Also, depth of field control is important. Sometimes you want a lot, and sometimes you want a little. iPhones use such short real focal lengths and small sensors that their depth of field tends to be very deep. As you move to larger sensors, for a given aperture and field of view, you will have a lot less depth of field. This can be desirable for many situations, and you can get even more control over it by using a tilt/shift lens.
Despite my best efforts, I can't get my iPhone to sync with my studio strobes. That's a big deal for a good portion of the photography I do. Photography, fundamentally, is all about light and having ultimate control over it(as you do with strobes) can be important for certain types of photography.
It's important to remember in all of this that the most important component is the guy controlling the camera. I'm active in antique watch collecting, and a lot of guys like to throw money at cameras thinking they'll automatically get incredible photos of their watches. I know one guy who has a D850 and a 105mm f/2.8 Micro, something not at all lacking in capability, and can't manage light something well enough to see the important details. Another guy has a high end Canon(not sure exactly what model) and can't focus to save his life. I see plenty who crank the sharpening slider all the way up and get weird artifacts. Meanwhile, I've seen another guy who earns his living as a full time pro photographer prop a watch up on a windowsill, maybe use a piece of paper as a reflector, and snap a photo with an iPhone that's better than 99% of what you see posted. When he actually gets out his good equipment-and more importantly sets up the lights properly(and he's told me he can spend a half hour playing with the right combination of light for a particular shot) his photos jump off the screen and often let you see something better than if you're looking at it in your hand.