For me personally, its for a variety of reasons

1: Targeted ads and algorithms and such are typically used to manipulate you to feel a certain way or hold opinions you may otherwise not have. This has been demonstrated and shown to happen several times, such as with Cambridge Analytica, and its pretty concerning. I want to see things for myself and form my own opinions, not just being manipulated to believe what some big tech company or advertisers or the like want me to think.

2: Just think about all the data a lot of these companies can and are collecting on you. For instance, if you're on a regular fully Googled Android phone, Google most likely has access to your physical location at all times. What possible need is there for this? Why does Google always need to know where I am? Just looking at it simply, its none of their business, and no justifiable reason for them to know it. There's no possible benefit or good thing that could come out of Google knowing my wheareabouts 24/7. If there's no reason for them to know the info, why give them it on a silver platter?

3: The data being collected is also usually handled very poorly as seen through constant data breaches of sensitive information and the like, and can also be easily abused in general. I myself have been personally targetted and stalked, and the stalker got mine and my family's information from data broker websites. Its pretty scary the amount of information these companies collect and share and make freely available about you, and it can be easily used against you.

4: Another example of the data being collected being misused is for example what's happening in China, with the social credit system. The social credit system basically determines what you can do and everything about your life, such as job opportunities and employment, access to finance and banks, ability to travel, and a lot more, based off a variety of factors, from things like what you post or do online, to even who you're friends with, and more. While you may argue that this is just China and there's nothing to worry about, similar systems to this are already being worked on and tried by US employers and companies, and there's nothing stopping things comparable to the social credit system from happening or being put in place in the West or elsewhere in this current surveillence capitalism world we live in. Something like this happening should absolutely concern you.

5: People have straight up had their lives ruined as a result of this mass data collection and privacy invasiveness. For instance, I remember hearing a story of a man who shared his Google account with his uncle. His uncle murdered someone, with his Googled Android phone in his possession, and Google provided the location data and such to the police, and instead the nephew was accused of the crime, and basically had his entire life ruined because of it, over something he didn't even do. Just look at what's going on now with abortion in some states in the US for another example. Its pretty scary to think about things like this happening, as it really could happen to anyone.

At the end of the day, these companies like Facebook and Google aren't your friends. Trust is earned, and I don't think any of these big tech companies earn it based off their actions and track record. What I do and how I live my life is none of their business or concern, and that's how I feel about it, and wish more people would see it the same way, or at least put some thought into it instead of blindly accepting mass surveillance and data collection.

(Hopefully this all makes sense and wasn't too rambly, pretty tired rn lol)