The idea that if you are willing to give up some privacy, the ads you see will be high quality and relevant. That sounds like a great trade and for twenty years I waited for those useful ads.
They still haven't appeared. When I consider what I trade for the ads I'm getting now, it's no where near good enough. My data is valuable and Google et al just aren't offering enough in return. The only place I ever see decent ads is on Google's search page.
I think the reason we see shitty ads like the belly fat ones, is because Google isn't actually trying to serve us first. Their main concern is ad dollars. So even though Google knows you watched a video on changing guitar strings an hour ago and now you are standing in a Guitar Center, instead of showing you an ad for guitar strings, you're going to see an ad for belly fat because that advertiser is willing to pay a fraction of a penny more.
So to answer your question about why you should care - it's because you are greatly overpaying for the service you are getting. Of course there are other reasons too - avoiding ads containing malware is one. Protecting yourself from tyrants is another. If you are socially or politically active, you may not trust that the government now or in the future can resist grabbing the data that shows you support abortion rights or attended a BLM or LGBTQ march or were part of the crowd on Jan 6.
Let me ask you this - if you learned the tracking is actually a person on the other side of the planet watching in real time, everything you do online, every conversation you have, every site you visit, and mix it with the data they can get from your phone (location, phone calls, music preferences, who you are spending time with, etc...) and they manually log it into a database, would that change how you feel about it?