Let's Play videos don't require as much effort as a "Funny Moments" video. It's part of the reason why the vast majority of gaming channels never get off the ground. Consider the following:
Would you rather;
1: Have a video you worked on for a week, be proud of, and make $1.
2: Make a video you took 20 minutes to make, care not for, and make $1.
Most would answer the second [at least in their head (but won't admit it)], but the first option grows faster as a channel, and will get to $2 faster than the second option. However, the vast majority of channels in general see potential money and fame as their objects, which is why they pick the second option over the first.
It's very easy to tell who chose the second option, just look for people who show up on the forum and plug their channel in inappropriate places each chance they get. Or, find unedited LPer's on YouTube who have a hard time continuing to talk and make good jokes during their videos.