I've been to tons of tourist spots and onsens in Miyagi by train/shuttle bus with no issue.
In Korea, because of the high car penetration rate for young people and new families, many attractions are inaccessible without a car.
You're just a tourist so you may can't see everything, but didn't you saw many small/family cars run on Miyagi at trip? Sendai city in Miyagi, maybe ranked TOP 10 city for great public transport in Japan, 75% of family still buy at least a one car. Rest of the place is far worse than Miyagi for transport. Of course they buy cars becaues it makes sense.
https://www.statista.com/statistics/1027885/japan-share-car-...
There are cars in Japan, of course. Families own one because it's convenient, of course. Also there are more cars in Japan than Korea, mostly because more people can afford it. But I'm quite sure that, at comparable income levels, Koreans own more cars per capita.
Korea has a very car-centric infrastructure, and people own massive cars, and drive recklessly, because the roads allow you to and people are used to it. Children are taught to never cross the road if they see a car, no matter if they have the right to cross or not.
I'm not saying Japan is a car-free country, of course. But car culture and car safety is a lot more reasonable.
Japan's car-ownership experience is rather hostile. Expensive tolls, Shaken inspection, and higher price tags than SK. In a way, this benefits tourists because streets are cleaner, cars are newer, and people tend to drive less.
In Korea, it's flipped. It's great place for car ownership. But from tourists POV, it's chaos.
As much as I love Japan, I'd prefer to live in Korea while visit Japan as tourist. Seoul isn't so bad once you go outside from central Seoul to new cities. Everything is more spread out, cleaner, and very modern.