If one is relying on a small number of videos, that's probably true (Wasn't it $6 per 1,000 views?), but a skilled composer should not have to rely on a small number of videos. In addition to ad revenue, YouTube has a donation feature integrated into their site.
A quick google suggests X per 1000 ad views (varies depending on ad demand, but the number I saw was $7.60), and the ads aren't shown for every actual view. And that's before YouTube takes between 30% and 45% (and before your YouTube Partner network's cut). Also depends on how much viewers engage with the ads, which I imagine would be quite less with music (because they'd run it in a different tab while doing other things).
People with millions of subscribers (not views, full subscribers) pushing out four or five videos a week that constantly hit million views or more, even with branding deals and referral fees, still aren't making livable wages and often need to work jobs on the side.
When I want to listen to a composer's work, I'm looking for specific songs. The *one* song out of the 20 or 30 in the game. Unless you made the soundtrack to Chrono Cross or Symphony of the Night, there's no way I'm going to listen to the entire soundtrack, even if the game itself was extremely popular.
I bought some music off Amazon about a month ago. Notice the lack of entire game soundtracks:
I'm not an audio person - I bought these for the purposes of adding to my game design folders. And I'm the abnormality, in that many consumers would've just pirated them. And I've rarely bought this much music before in my life (again, not being an audio person), so it was an almost one-off occurrence.
In many games, I don't care about any of the music in the game enough to pay for it. I'm sure similar applies to other people who listen to music on YouTube. What are they even supposed to google for, "Song that played during level with the tower in Game Y"?
If you made One Wing Angel, it might bring in some ad revenue (ignoring the 200 YouTubers who'll post your music on their own channel giving you no revenue). But even then, for one of the most popular videogames of all time, that's pretty much the only song people remember. Sure, they'll recognize the tune of the others if they listen to them, but they won't go out of their way to listen, and you'll get virtually no ad hits for them.
Bands do a bit better. They have a popular song and then people say, "Hey, I like this band." and check out the other songs. Much much less so with games, I'd guess.
All of Halo's soundtrack might as well only be Rock Anthem to Save the World. The average fan doesn't remember anything else, and the average consumer doesn't even remember the name of that one.