do they have a serious mental disease?
more common than you think at gyms 
Nah, that's pretty common in many of the gyms I've frequented.  One country club kid of maybe 17-20 used to be training biceps when I'd come into the gym and he'd still be slaving away on them when I left.  I felt bad for the lad, 
all no homo.  He was supersetting from one thing to the next, constantly going to failure, even using good form.  But the kid had a build I would've been embarrassed to have when I was 14 and maybe six months into training seriously.
15+ hours/wk. might be a hair extreme, though ... best I can tell, most of the skinny guys or fellas who have been stuck for years typically don't train much more than 12 hrs./wk. (if of course you could call it "training"; a lot of them bench for hours, curl for ages and do tons of overhead presses in a Smith, then do a few token sets of pulldowns and leg extensions).  
I don't personally know anyone who actually trains more than 8-10 hrs./wk. and has any kind of size, including guys who have been on for 10-20 years.  I make my best gains training about a third that TOPS, but I am starting to get old and have a lot of intermittent joint pains now.  The days of awesome upper body gains from Heavy Duty-type 35 min. workout 2-3 times week and skipping legs might be over ...  more of a pumping-oriented routine might be indicated, 'specially since I'd like to be in this past 40 

But I'll skill skip legs unless I want to compete again (naturally gifted legs, especially calves.  Combined with small ankles, my 17" untrained calves look better than some fatass permabulker's 19" ... and if I start training them again ... mwahahahahahahahahaha 

  They blow up even natty, thank the Old Gods and the New).