It's good to ask "why not shoot the tyres? Why not shoot them in the arm, hand or leg?".
But it often also means the person asking the question isn't aware of the reality of shooting somebody.
Let's take the case of Usman Khan in the 2019 London Bridge attack. Khan appeared to be wearing a suicide vest and despite the fact firearms officers shot him around a dozen times Khan was still alive...able to move. Shot in the head yet still able to sit up minutes later.
This shocked professional police firearms officers:
https://www.bbc.co.uk/news/uk-england-london-57354874
Turns out it's not necessarily straightforward to kill somebody with multiple firearms. Never mind one.
So, shooting somebody in the arm, hands or legs ISN'T going to reliably stop them from shooting innocent people or detonating a bomb.
The policy is shoot to incapacitate. Not shoot to kill. Khan refused to comply with police demands to begin with, so couldn't be arrested.
Likewise, shooting a tyre or two won't reliably stop a vehicle. But shooting the driver is more likely to stop it.
In situations where a suspect is refusing to yield to law enforcement or soldiers such as SAS, it is authorised to fire weapons at them. If there's a weapon or honest belief they're armed in some way, it's authorised and legal to shoot them. They can shoot until the suspect is no longer a threat. Now, that doesn't necessarily mean kill them, but it is an option available depending on the circumstances.
In the majority of incidents firearms officers are called out to, it is resolved without police firing a shot. A minority of incidents end with the suspect injured by police firearms. A smaller proportion still ends with a fatal injury which isn't very common in the UK.
The policy is shoot to incapacitate.
Shooting a suspect in the arms, hands or legs not only DOESN'T reliably stop them from carrying out a threat, but it is also very difficult to hit a small moving target like those.
On top of that, it's possible for a bullet to pass through hands, arms and legs to travel on to kill an innocent bystander. Olivia Pratt Korbel was killed when a bullet hit her mother's hand and travelled on to kill her.
Bullets don't usually travel through the torso as easily, so that's another reason to shoot that area. The other reason being is it's easier to hit a moving target that's big like the torso.
All that said, sometimes police operations go wrong and the wrong person is killed. Police operations involving firearms will never go perfectly as there's so many variables at play.
Or an officer fired a shot just as any perceived threat had ended which the prosecutor Tom Little KC was suggesting in relation to the shooting of Chris Kaba.
But it is not as simple as shooting a suspect in the hands, arms or legs. Or even shooting tyres which is also a small target in relation to the windscreen or driver.
These situations are fast moving and complicated.