It's a combo of breed and owner. You have a dog with aggression either not systematically bred out from catch dog or fighting origins, or deliberately bred in to make the dog look well hard. Then you have the sorts of owners who want a dog like that.
I met a much-tattooed, shaven-headed bloke a few years back, walking a young dog who was came corso crossed with iirc staff. We had a friendly chat, and it was obvious that he understood dogs, had a purpose in mind for the dog, was working on socialising her, had her walking well on a lead and so on. I very much doubt that dog will ever be a problem. She probably has the capacity to be (big, strong, guarding instincts from the corso, possible dog aggression from the staff) but the owner understood what he had on the other end of the lead and how to manage her.
Then there's the bloke I encountered the other day. He already had an Amstaff (as he described it) of about 18 months with zero recall (hence on lead, at least) and poor lead manners. He'd recently acquired a bloody huge XL bully, turning 5 months old and already looking as if it weighed about 30kg. So far, goofy and puppyish and recalling well, but the bloke clearly hasn't the ability to train a dog who decides it doesn't like recalling, and won't seek help. If that dog has any dodgy genetics or develops any issues, it is going to be a dangerous nightmare.
What's the solution? Maybe a ban, though ultimately that will be bred around. Maybe a system where dogs of a certain size or type need to licenced the way that shotguns and rifles are, including, as with firearms, a check with your GP, a check for a criminal record, a home visit etc and regular renewal checks.
And if any type of dog appears and starts causing regular fatalities, it gets added to the list.