Our economy is intertwined enough that "your actions somewhere down the line may be making it easier for someone to make choices that ruin their lives" is probably true of literally every single person. If you work on a product making low-skill hiring more efficient, you're also lowering McDonald's costs and making it even harder for people with unhealthy relationships with food to resist, and thus "you may be ruining some people's lives".
It's so broad a claim that it's utterly meaningless.
If you started your own site there's absolutely no way you could be profitable competing on both hosting costs and payout to creators.
I broadly agree that creators hugely benefit from these platforms, but that really doesn't seem to be what they're optimizing on imho. And you basically admitted they're a monopoly with that final statement.
Having a low cost infrastructure for video streaming and good advertising deals is not a monopoly, it's an investment.
Advances in food production means that the proportion of people starving and undernourished is lower than ever. It also means that, for the first time, excess food is a problem for more than just kings. It would be kind of crazy to complain that these advances in food production are per se a bad thing because they enabled obesity. They're just a tool that accelerate both good and bad uses.
It was already possible to feed the idle, addiction-prone part of your mind with channel-surfing and People magazine. The Internet has accelerated this possibility by making everything about access to content more efficient, but describing efficiency solely as "making access to bad stuff" more efficient makes no sense.
To me it seems similar to blaming an advance in fuel efficiency for making it cheaper for bank robbers to get to their targets. It's true, but also missing the point.