Buzzkill

While I'm not an expert on the topic, I think the inspiration for using chainsaws as weapons in video games comes from the classic slasher film "Texas Chainsaw Massacre."  I'm not really a fan of the long running series, but I've seen bits and pieces...chunks (if you will).  It's interesting to note that the original movies is surprisingly lacking in on-screen gore.  Sure, some blood gets splashed about, but the murder, Leatherface, is given a somewhat tragic angle in that he is a mentally disabled man browbeaten by his psychotic family into being the human equivalent of an attack dog.  He can't even speak using actual words although he does have three different masks in the first film which he wears as a form of self-expression.  Leatherface also wields a hammer as much as his iconic chainsaw, which is a lot smaller and more practical than what appears in the sequels.

Take the word "practical" here with a big grain of salt since chainsaws make for cumbersome melee weapons.   They're heavy, bulky, imbalanced, and generally speaking, don't start up on the first pull.  Until their little high performance 2-stroke engines are warmed up they usually can't be throttle up with out the motor dying.  What's more, the spinning chain can cause bounce-back if it's brought against a surface too quickly, putting the operator in a lot of danger.  On top of all this the teeth on a chainsaw are designed to cut through wood and not much else.  As such they tend to get caught up in clothing fabric.  They also dull very quickly against hard materials and as such can't tear through most metal surfaces (a fact that is well demonstrated in the original film, but largely ignored in later iterations).  Then again, if you're mentally handicapped, it could very well be that none of these facts would make an impression on you're weapon selection process.

Nevertheless, video games have embraced the chainsaw as a deadly melee armament; Doom, Gears of War, Splatterhouse and most recently Resident Evil 7 are just a few examples.  It's not hard to see why those games chose the chainsaw over more widely used hand-to-hand weaponry.  Chainsaws, to the uninitiated, are incredibly intimidating with their loud, shrill-sounding motors that billow for clouds of oily vapor.  It's definitely a given that games thriving on gore like to show the effects of being sliced and diced in a less-than-expeditious manner.  Again, it's not my thing, but it get what the designers were going for.

Of course what makes sense in Texas Chainsaw Massacre doesn't necessarily jive in other settings.  For example, the Sawyer/Hewitt/Slaughter family patriarch is (as far as I can tell) some kind of undead.  Scary?  Sure.  Plausible?  Not really...
 

What's that?
 I can't hear your criticism over the sound of my CHAINSAW!