If anonymous billboards or banner ads can convince you you aren't good enough, your life is probably not great with or without ads.
If an ad convinces you to, say, get a gym membership or go on Ozempic, who's to say what happens next? Maybe you do start feeling better about yourself.
They don't consciously convince you directly anymore than a slot machine convinces you to give it one more spin - it's done on a subconscious level. For instance one of the most famous, and effective, ads in history is Apple's 1984 ad. [1] A 59 second ad where the only mention of what's being sold at all happens in about 1 second with a reference to a brand name and then a logo. See: ELM model and peripheral processing. [2] And this is all day one advertising stuff.
Advertising is a horrific industry. It probably always was, but at the modern scales, it's outright dystopic. I think there's simply a large amount of cognitive dissonance around this issue because advertising drives the paychecks of a whole lot of people, and it's rather difficult to get a man to understand something when his salary depends upon his not understanding it.
While I agree with your point about understanding, I think there's also an issue of self-image. "What? Me? Influenced by some ad? Get outta here! I make my own decisions!"
If an ad convinces you to, say, get a gym membership or go on Ozempic, who's to say what happens next? Maybe you do start feeling better about yourself.