People boycotting certain products only really works if an alternative is available and attainable, or the demand is elastic.

For example, if I go to any grocery store, all the pasta, rice, buckwheat, bread and other staples are packaged into single-use plastic, as are hygiene products like toothpaste and shampoo. I have no choice but to be part of the plastic waste problem since there is no alternative and the demand for food is not elastic—I literally can't go without food and basic hygiene.

But I can and will avoid buying problematic products like teflon cookware, fast fashion, ICE vehicles, tech products with severe privacy/ownership/repairability issues since there are alternatives available and if not, I can go without since eg Alexa smart speakers are not essential for life.

Hence, we need to hold companies, whose products are problematic while not having alternatives and that are essential for life, responsible and force them to change to less problematic practices. In short, eg single-use plastics should be regulated out of existence wherever possible.

And for products that have better alternatives, we need to raise awareness about them and raise their social acceptance/desirability (make them cool). Plus we need to increase their availability and attainability—what use of is an ethical alternative product if it's not easily available in my country or if the price is not affordable to everyone who can afford the "normal" version?

Indeed

Claiming that oil companies are to blame for producing all that oil seems stupid. If you use less oil, they make less oil

That's not really what OP is saying though? They're talking about corporate efforts to make it seems like the consumers are the problem, not them, and many are still falling for it. As long as the awareness of this is not raised and more people aren't pointing fingers at the corporates the whole don't buy their product is never going to be effective, same for advocating for regulations (rather, especially the regulations). You're assuming everyone knows the root of the problem already, but that's just not the case here.

This is a frustrating kind of defeatist attitiude I'm finding is getting more and more common.

It comes from a place of unwillingness for personal and habitual change. It's hard to accept that we all have to change our lifestyles and accept that how we're living is going to have to change. That there is exists some scenario whereby we all continue living exactly how we're doing now with the same consumer behaviour and expect a bit of regulations to change everything. Or delay changing until after these regulations are in place, when in reality BOTH needs to happen.

What's the point in sitting on your ass complaining about the behaviours of other individuals and organisations when the only thing you have direct control over is your life.