"Why do you watch a playthrough of a game? Just play the game!" I 
have heard that plenty of times, here are some of the reasons why I do 
not play the game that I am watching:
1)
 Lack of time: This is an oxymoron, I know, but the case here is that I 
work, do other stuff, I sometimes do not have the time to directly 
dedicate to a game as much as I would like, the whole nine yards, while 
it sounds stupid to say that I could have dedicated the 20+ hours 
watching the game to actually playing the game this takes me to the next
 point;
2)
 It's just background noise: Yes, sometimes I am not even watching the 
thing, I just have it as background noise and from time to time turn to 
it, while this sounds weird to some people I rather have this as 
background noise than music (yeah, sue me on that). I only alt-tab or 
turn to another monitor when I hear some commentary on something that 
could be potentially important (or stupid, maybe something cringe 
worthy).
3)
 Do not have the resources to play it: While I have several computers in
 the house none of them, well maybe one of them, are built on the last 
decade (2000-2009) which leaves me out of playing most of the games I 
could potentially be interested in, I could surely invest on a new 
machine but I also have to pay rent, utilities, a car, food, etc. So a 
new computer is at the moment out of the question until I can either 
find a good enough discount or manage to scrape enough money to build 
something out of parts I get from flea markets (of which I already have 
half to build a decent computer).
4)
 Do not want to mess with emulators or old hardware: Now looking at the 
other way, I have enough old hardware to mess with and play some really 
old games straight from the CD or Floppy, even from 5 1/4, but going 
full retrogaming means I need a CRT (which I actually have stashed and 
needs to be checked by someone who knows how to fix a CRT because it 
shorted out) and an old keyboard and trackball mouse (of which I have 
all), did I mention you need an old PC for the whole experience to be as
 close as possible to how it was back in the day? Most will not try to 
figure this out, no matter how cheap the parts could be, and sometimes 
figuring out an emulator can be a pain to some (and Linux doesn't make 
this any easier apparently).
5)
 No demo available for the game, not willing to spend money on something
 I can't try out: This is probably the main reason I watch most new 
games on YouTube first before buying them, while I could make a risky 
decision of paying 50+ dollars for the latest whatever game or 20/30+ 
for the latest indie game I do not feel this is not a sane choice, this 
hits harder in a time in which demos are no longer available, developers
 barely give a hint of the game through their available material, piracy
 becoming a more viable option before making the final choice, there are
 so many more reasons that require demos to be back on the table, 
urgently, so that we as players can either dismiss the game or build a 
more appropriate hype around the thing that we are looking forward for 
(perfect example, Alien: Colonial Marines was an obscene kick in the ass
 to everyone who was looking towards the full experience of reliving 
those very tense moments only to find this carcass of a game).
6) Kickstarter/Indiegogo
 campaigns banish into thin air: Not so much as a playthrough but I 
mostly end up finding out about games that become vaporware from 
YouTube, testers and the like, sometimes I wonder what could have been 
of a game had the developers not dropped the issue for whatever reason 
they decided to do so or not scammed players (remember that game in 
which you would become an Ant, another perfect example of what happens 
when you have the wrong people in your team and then get robbed of 
everything).