most arcade machines these days just PCs (most run linux actually with a few running various version of either win95 or win2k)...
It's just far far cheaper to manufacture and there is literally no design work involved

Not to mention the parts are more reliable and you only need to test the software really, repairing and replacing parts is a breeze too.
Believe me I've had my head inside pretty much every machine you find in an arcade...
DDR/ the who Bemani series is a PC with a custom linux based OS
Most photo-booth are pentium based linux pc
midway games like hydro thunder, Arctic thunder etc are all PC's running win 2k
Beach head 2000 is a PC running win 95...
They still make games with custom hardware (I believe most namco and sega games have been up till now) but lots and lots of new machines are just off the shelf PCs... Definitely makes it easier to repair, that's for sure.