Historically desktop computing has been about giving people a tool they can use to control the environment, whereas the smart phone is a tool that brands use to control you.
The mindset gap between those two things is much larger than the technical gap.
As for technical issues the devil is in the details. Often my hackathon kit has been a cheap android device with wireless keyboard and mouse. With an a ssh terminal and something like an RDP or VNC client you can do all kinds of software development. In fact many firms (e.g. the Bridgewater hedge fund) like to centralize people's desktop environment and if you do that a tablet is an option.
My understanding is that iOS devices don't support the mouse, which I don't get because mouse support "just works" on Android.
People expect a different software environment on the desktop as opposed to mobile. For instance, Windows can run on a tablet environment but it has a hard time being competitive because the Windows operating system is much larger than other mobile OSes.
It may be true that "Windows is bloated" but more to the point, Windows has more features than Android or iOS. For instance there is Hyper-V virtualization, Windows Subsystem for Linux, and Microsoft IIS. Most people wouldn't care about those things but some of us do and you wouldn't have "it runs as Windows" as a selling point if they hacked away a lot of functionality.
As for Apple, they have very profitable product lines in both the iPhone and the Mac. When you are expected to make money hand-over-fist the way that they are, they are not going to take the chance of raising the average selling price of a phone from $800 to $1200 by merging in "laptop" features and then lose to opportunity to sell you a $800 mac mini and a $1800 macbook.