Why does everyone call it hurth, instead of harth?
Hearthstone is a game, and everyone pronounces it properly. People know what a hearth is, they also know how to pronounce it. But for whatever reason no one says it correctly when used as a verb, even when they know they are mispronouncing it.
They probably do it just to piss you off, specifically.
It’s VC not DM the same as it’s hurth not harth.
The only person I’ve ever heard pronounce it “hurth” was from Alabama. And, well, you know . . .
Hissss
uh oh, you found the one that can read! watch out!
Hahahaha 
I pronounce it “hear th” just like I pronounce heart “hear t”.
I know how its supposed to be pronounced, but Im doing it to bother people.
I also pronounce gadgetzan “gad jetson” to crush the spirit of my english majored friend.
I do not, however, pronounce .gif “jiff” because im a jerk, not a philistine.
Wait we don’t pronounce it heathstone?
Same reason people say “sham-in” or “shaymin” instead of shaman.
I love how this bothered you enough to make a post about it.
For the same reason people pronounce the h in herbalism.
It’s pronounced hee urtth
It’s pronounced HueHueHue. Especially if you are a paladin and you bubble-HueHueHue.
Hurthstone.
I’ve been saying it like He-arths-tone
There is a famous streamer who pronounces Hearthstone “Hurtstone/Hurthstone.” Could have something to do with it.
It’s possible, though my money would be on the fact that English is not a phonetically spelled language, and so we don’t always know how others pronounce words that we read, until we hear them say it.
It is pronounced Hurthstone