Because the majority of users and developers have no idea that websites can even be keyboard navigated. Of increasing annoyance is how many browsers now no longer even navigate anchors by default for the same reason, instead only focusing form elements. (
Even if in both cases it's a violation of the WCAG that could get you in trouble legally.
I ALWAYS give my anchors and other elements the same state for :focus as I do :hover... most people skip this out of ignorance. What's REALLY annoying is the people who turn off outline on :focus, as that's just willfully telling keyboard users to suck it.
Though if you have strain injury, you should consider a pointing device -- like a thumb based trackball -- that immobilizes the hand. For most carpal, repetitive stress, etc, users trackballs like the Logitech M570, MX Ergo, Elecom EX-G, or even the Perixx Perinence-517 work wonders. Amazon even has a rebranded version of the Elecom they sell under their own name for around $24 that's fine-and-dandy.
It takes very little strength or exertion to move the ball meaning you don't develop pain across the entire hand like you do with a regular mouse. There is one hell of a conversion time at first, but once you get used to it? Well, it gets kind of hard to ever use a mouse or trackpad ever again because they suck so bad.
... and they also make amazing TV remotes if running a media center PC.