They aren't. Wacom is a hardware partner for Microsoft and MS aren't keen to push any of their partners out of the market. What they are trying to do is offer premium devices to the market that address needs that aren't being met by their OEM partners.
In many ways it's more about a way to alter and enhance the image of the company than a flat out play for marketshare and revenue. For example,
http://www.fool.com/investing/2017/01/04/microsoft-corporations-surface-studio-is-exceeding.aspx
MS knew it was deliberately targeting a small professional market with the Surface Studio and if that is to be believed they originally only ordered ~15k units for 4Q 2016. It evidently proved far more popular than their estimates. However, they still only increased their manufacturing orders to 30k units.
In the same way that the Surface Studio wasn't meant to push out any of their OEM partners but was introduced to the market because none of their OEM partners was addressing a relatively small but wealthy niche of Windows users.
In this case, drawing/art is a pretty small niche of personal computing. And the Surface Studio addresses a niche within that niche that wasn't adequately being addressed by anyone. The purpose wasn't to directly generate gobs and gobs of revenue (Surface Studio is projected to bring in less than half a percent of additional revenue for the first year). The purpose was to show the world that PC's aren't just boring business machines but something that can be adapted for use for just about anything with the right design. And that you can do so without going into a race to the bottom, IE - to offer a premium quality device.
By doing that they widen the appeal of the overall PC market (or at least its visibility to non-PC users) which may indirectly lead to greater revenue than that generated by directly by Surface or Surface Studio. Which is why they are more than happy if their OEM partners flat out copy their design (some of the Surface Pro competitors from their OEM partners are almost identical copies except using different materials) and actually encourage their OEMs to copy it as much as they want.
In fact, recently Microsoft had this to say when speculation that Dell was copying the Surface Studio with their Canvas product and that Microsoft were angry and ready to sue.
While I've heard many wonder aloud whether Microsoft officials are angry -- to the point of possibly suing OEMs which release near clones of Microsoft's own Surface hardware, Microsoft's chief marketing officer Chris Capossela recently told me and my Windows Weekly co-host Paul Thurrott that this isn't the case.
In building its own hardware, Microsoft's number one goal is
"to create a new category of device and expand the totally addressable ecosystem," Capossela said. He added that Microsoft "expects" its PC partners to follow Microsoft's lead in these new device categories.
From
http://www.zdnet.com/article/dell-just-released-a-microsoft-surface-studio-style-display/
He goes on to mention that its OEM hardware partners (which would include Wacom) are far more capable of offering a diverse array of hardware options than Microsoft can.
While they do offer devices to compete against Apple devices to some degree, they aren't interested in becoming a hardware company like Apple.
Regards,
SB