Because many people can do everything they need on their undocked phone already.
For me, my phone is an auxiliary device and my preferred device is the computer. But I'm sure I'm in the minority.
Garmin has made a brand of themselves doing this. Every sport activity, there's a running watch, a golf watch, a whatever the hell watch, a gps this, a gps that, a bike computer, whatever, all bespoke and siloed for a few activities, when really they could just sell you a single slab that connects to a gps and could do whatever. But then, they'd only be selling you one product and not dozens.
This is a nonsense, how is my phone going to tap into a central server at my home to do the image processing of taking a filtered video while on holiday? Stream MBit/sec over international cellular networking at my cost? Why would I want my car to tap into a handheld dock which I have to remember to bring with me and means my home computer is unusable by anyone else for the duration of my journey? How is my watch benefitting by having a long range wireless radio constantly running instead of a low power internal processor?
But at the same time, if I'm on a boat and trying to look at charts and trying to find the fish, I don't necessarily care about it being small and light, I want it to be pretty big, and it needs to plug into the sonar fish finder and depth sensors on the boat.
I want that running watch to be pretty tiny, so it can only have a tiny battery. It doesn't have much of a display, so it doesn't need much graphics capabilities. Since it's barely interfacing with other devices it doesn't need a lot of peripheral support. It's not really going to load a lot of data files and doesn't need much RAM.
That fish finder is going to run a high resolution display as it's main duty so it needs a decent GPU. It's probably going to be connected to the boat's 12V system, so it doesn't need to be concerned with power usage. It's going to do a lot of reads from data files and need to hold chart data and overlay other information. It needs a lot of connectivity options for all the accessories I'm going to hook up to it.
In the end the needs for the compute are pretty different based on the different tasks. I don't want an ESP32 as a desktop and my watch doesn't need dual 3080's and 128GB of RAM.
Every year I'm more surprised this isn't happening more. I get that the empty clamshell laptop you slide a cell phone into never took off, but this just gets cheaper and cheaper every year. The drivers exist, the OS capability mostly or entirely exists, we're just... not doing it. With a dock, you can cobble things together; the TV your dad threw away, a keyboard you found at goodwill, any old mouse. Incredibly accessible; anyone with a phone and that level of access to tech could have a full computing environment.
I have tasted this with my Steam Deck. I have some limited uses for it when I take it places but want it to do just a bit more. I'd love it even more if my cell phone, which has all the computing power it needs for the uses I have, could do it. If I was a student, or less well off, being able to turn my phone into a full computing environment and use it at its full power would be a great way to save money. The only two limitations on cell phones at this point are 1. the IO with the screen and touchscreen and 2. power limitations from being stuck on battery. Other than that they'd be quite credible laptops from ~2013 or 2015 no question, possibly even a few years later. Very capable nowadays.