Notoriously bad? Compared to what cities? It's certainly one of the better ones in Germany, if not even the best. You can absolutely get around in Berlin without a car.
The other larger cities and conglomerations in Germany don't compare favourably, with Rhine-Ruhr (which stretches roughly from Cologne to Dortmund) being particularly bad.
In Europe Berlin's public transport system probably is only rivalled by those in London, Paris and Amsterdam.
I've been to berlin about five times and I've never used a car or taxi. I'm not afraid of walking so I walk to the hotel from a public transport stop, most I've experienced is around 2 km.
While there I find riding a bike a superior means of transportation. You can even rent electric ones and Berlin is _flat_. Also the public transportation works well. I've not been to Rhine-Ruhr area but down south around München it's not so good but still passable, I didn't need a car there either. It was a bit slow though when going far like München - Erding.
Amsterdam is pretty good although I wish the trams would run a little longer in the night. There are always buses and rickshaws though.
London I wouldn't even dare to drive, they do it on the wrong side! But I've never needed a car there either.
It's far from the German best, but it's still more good enough, especially considering how it is plagued by cheer scale and all the construction sites all the time.
You can get around in Berlin without a car, you're absolutely right -I've done it myself- it's just not convenient compared to using a car (or bike, if you're traveling light and reasonable distances), which is why, I believe, many people are using their cars despite the fact that public transportation exists. I very much prefer Hamburg's public transportation for speed, reliability and upkeep (though they often sucked at planning for rush hour and large events when I was living there), and the (albeit limited) experiences I have in Frankfurt, Dusseldorf and Bonn were better, too.
It wasn't my intention to insult anyone's home town, Berlin's history explains a lot of the situation, which I mentioned in my comment, so please don't take it personally.
Maybe you have a much higher level of stress tolerance than me, but driving though Berlin is anything but convenient.
And unless you are driving at night or the early morning it's also not faster than the S or U-Bahn with the added hassle of having to find a parking spot.
Possibly regarding the stress tolerance, the fondness (or lack thereof) of Berlin's specific culture likely plays a role as well.
And I'm not saying that the car is superior for any and all connections, but that it's superior for many that aren't easy to do by PT. It's similar with trains: Berlin to Hamburg is easy by train, but if your destination requires a switch or two, the car is often the quicker option on medium distances.
My partner drives to work by car from Fhain to Reinickendorf because the PT connection is not great and it takes more time. So that totally makes sense.
For me commuting to work into Mitte is 20 minutes with PT with one change at Alexanderplatz, which is super easy. So it depends i guess.
Berlin's public transport was perfectly fine for me just like a large city such as Amsterdam is (which is where I live). If it adds anything of substance: when we were in Berlin we were on babymoon, my partner was 6 months pregnant at that time. Also, I've been in Berlin on several occasions. As a Dutch person, I find German people rather friendly, with a near-excellent detail to service and hospitality.
Actually, the worst part of the whole travel was the way back with the ICE. The part in Germany went great (apart from having terrible LTE connectivity but I also saw loads of forests and such so that makes sense). It just stopped at the middle of nowhere, in Bad Bentheim, in December, with snow and all. The train wouldn't go further in The Netherlands. The DB allowed us to remain in the train with the heating on while NS (Dutch Railways) got another train instead which took an hour to arrive, was less luxurious as ICE, and was full. At the same time I caught a flu on the last day which was setting on during the train travel. Together with all our baggage, and a 6 months pregnant partner who I don't want to carry much. Fun times...
OK, one more point to make: German people are attached to cash money instead of cards. So sometimes I could not even pay with card. I like it, in a way, because it adds to privacy, and there's a nostalgia feeling, too. But if I'm used to card, it also feels like a step backwards.
The other larger cities and conglomerations in Germany don't compare favourably, with Rhine-Ruhr (which stretches roughly from Cologne to Dortmund) being particularly bad.
In Europe Berlin's public transport system probably is only rivalled by those in London, Paris and Amsterdam.