I think the key there is "all the time" - when we lived in the middle of a European city we used car-share, and the car seat was a bit annoying but since we were using it maybe once or twice a month tops it just wasn't that big a deal. Though we were using the seatbelt mount, and not ISOfix.

I guess if renting was big enough there could be option to rent a car with those pre-installed

That's generally an option, but it's not exactly cheap!

I looked into carseat rentals about ten years ago, and it sounded like it's only really common at the agencies located around airports. Overall, I got the sense that there's a lot of fear and liability tied up in it— that giving someone the wrong seat or installing it incorrectly or not cleaning it adequately were all potential landmines waiting to happen.

They even have inflatable ones you can easily fit in a backpack - I have one sitting in my garage from years ago when my tots needed them.

But yeah, if your kid is too small, it's a hassle to to carry around that bulky infant seat.