They are 60w chips so very much smack in the Mobile & SFF power usage range, ARM's biggest strength is their efficiency at lower wattage ranges, it can scale up very well, Amazon's Graviton series server chips and Nvidia's Grace CPU's are awesome examples of this, but you then start playing into x86'd home turf and it becomes a much more level playing field.In many ways they are. ARM's currently at its strongest in mobile, and the clear majority of Mac sales (as with many computer brands) are laptops. Apple either thinks it can scale up to workstation performance or is willing to make a tradeoff knowing that its core sales will be stronger. Apple's market share is up even with only the M1 Macs in play; that could go up further with the new MacBook Pros, and any growth in desktops will be icing on the proverbial cake.
I am sure they have a desktop-class workstation chip in the works something ranging from 95-120 watts, but at that form factor many of the design decisions that they have made for the M1 chip make far less sense because that is just too much to cool in such a small space, you are going to want to separate that CPU and GPU. Because up there you are going to be wanting to pair a 120w CPU with a 300w GPU and having that packaged as a single SoC is just a good way to make a big mess unless you get really fancy with cooling solutions which gets needlessly complex even for Apple's tastes.