In my case, I'm also very careful. But I use a case because it makes it easier to handle it, otherwise it gets slippery.
But the main reason, it is to protect it from other people around me who may accidentally drop it or damage it.
Also, the Spigen case I use has something that allows me to make it stand.
There's a divide with case users (I've no formal study or numbers, just based on observations and conversations here). Some are quite careful (as you are) and others admit that because they use a case they willingly toss their phone around. Near as I can tell it's a fairly even split.
I don't hate cases, I just don't like them for my own use. I have used them in the past for specific things and will in the future for those same things. If I had a physical job I'd be using a case. But I do not. I've sat behind a computer all day for my employment since 1999 and since 2020 I work from home 100 percent.
The majority of the time my phones go into business card holders. I have one for each space I occupy. My desk in the front room, the desk in the bedroom next to the bed, in the garage where I have another setup, etc. In cars, my phone goes either in a cupholder or in a space before the shifter in the case of my older car. When I'm out the phone will go on a table or on my seat/chair under a thigh so it doesn't move. When walking, it's in a back pocket.
I handle my phone with two hands. Explaining to my wife how I damaged a ~$1000 phone because I one-handed it is not on my list of favorite things to do. If I'm in crowds, I cradle my phone or put it away.
I have two kids, one is 19 and has learned to protect his phone and doesn't use cases either. My other kid is 14 and since I got her a new phone earlier this year I made her put it in a case. I can't trust her to not drop her phone. My wife uses cases. Aside from the fact that her job sometimes requires physical activity she'd still use cases. It's just her thing and she likes the whole design of cases thing.
When my kids were younger they were taught not to touch our stuff. They had their own toys and eventually they had their own devices so going after ours became less of a thing. My daughter managed to ban herself from using my iPad when she was 10 or so because she put a dent in it. So, stuff has happened. As careful as I try to be, things do get damaged. But in general, being as careful as I can has led to devices being undamaged.