For me, there are several very different good reasons. I write Them down in order they come to my mind not from imoortant to less important or vice versa.

1. i dont want to be judged before met in Person

What do companies do before they hire you, banks that you ask for money etc. They look you up in the net before they meet you. Think they know you. No Chance to present yourself as you want it. Yes you can polish your social media but not all the hidden profiles they made of you...

2. stalking

i once studied in a different country. Had 3 dates with someone i only realized then is weird and kindly ended dating. Then this person was stalking me for a year and only stopped thx to the police intervening after a lot of efford from my side. Luckily the person only had my email and phone contact and new which city I came from. I was SO glad Ive never been much on social media and stuff.. Cause this person would have come to my hometown, waiting in front of my university or whatever - even wrote so. But couldn't find out. Most of my friends back then had their address / favorite bars / university courses favorites places and everythign in the net. I was so scared back then i would have changed my entire life to be sure not to Meet this Person. Luckily i didnt have to cause my life was private. If you think stalking is very rare and you will not be affected look up the numbers. Its horryfying. For woman especially but also men. People dont talk about it but During that time when speaking to friends i suddenly knew 5 other people who had to deal with it already. Also nowadays these apps exist were you can take a picture of someone and search for that face in the Internet.... A stranger can do that and find all your photos/ social media etc and be there in your favorite bar every nicht.. Scary.

3. data is power.

On the " i have nothing to hide" thing. Think of Google maps. They dont care where you go at all but by knowing where everyone goes they can predict traffic, give u alternative routes and theoretically they could also causetrafficb jams... :D while maps is useful they can do the same with any information. they are not interested in your political views but by knowing everyones political views they can predict and also direct them. This isnt theoretical its used in elections already. There was a scandal a few years ago about brexit ithinlk.

4. i like to have control about myself

Sometimes i might even agree to share some data for something else but i am forced to it all the time and i have no control whats done with the data to whom its sold next etc. But its still my personal data ...

5. governments/ laws change

Well i am german, actually from an eastgerman family. If my family teached me anything its that political systems can change very easily. My greatgrandparents, my grandparents, my Parents and i grew up in 4 different political systems, well actually 5 :D. Every system of those had different views of who is good and Bad And it was better they dont know everything about you...