There have been a few attempts to sell ARM-based laptops, including Smartbooks (1) and the Microsoft Surface. They've all flopped horribly. This doesn't mean they can never be successful, but it does make PC manufacturers less likely to take the risk again.
Of course, users can always buy an ARM tablet and add a cheap keyboard/case combo.
They needn't necessarily be slow -- see the Apple iPad Pro, for example. However, most buyers are probably looking for something that costs less than a Windows 10 laptop, and you can now get Windows laptops or 2-in-1s for $199 or less. These also have almost-instant on and can run sandboxed smartphone-style apps, albeit only a few hundred thousand of them.
In sum, it could be tough to launch an ARM-format laptop (much more expensive than launching yet another Windows 10 laptop), and the margins would most likely be thin or negative.