Why don’t they make tread patterns deeper, so that I don’t have to change tyres so often?
It‘s a matter of “cost-benefit analysis”.
If the tread pattern had an additional 20% added, it would not give an extra 20% tread life. This would be at the cost of extra fuel consumption, and a lessening of stability of the tread elements in the contact patch. The extra tread depth would wear away at a faster rate than when the tyre is part worn. The car wouldn’t handle as well.
Tread pattern grooves vary in depth across the crown of the tyre. This is because the contact pressure in the shoulders is normally higher than in the centre of the crown, so the shoulders wear faster. To have the tyre surface all wear out evenly and at the same time, which makes the customer happy, there is some variation between groove depth shoulder to centre, or there is more rubber there to compensate for the faster wear.
But there’s another aspect. Tyres are made to the same design standards all over the world. The whole world would have to change, and the most interesting aspect is that the extra rubber would affect the diameter of the tyre, because the casing would have to be made smaller in diameter to maintain body clearances. The tyre rolls around the diameter of the steel belts/nylon overlay, not the surface of the tread, ( which is why there would be more movement on the deeper tread, and thus faster wear) So the car engineers would have to adjust their standards too. It’s all too hard.
Convinced? Well, truck tyres offer a variety of tread depths, dependent on the position on the vehicle. Deep tread drive tyres are fairly common, their advantage being where we started- they don’t have to be changed as often. However, the same tyre is entirely unsuitable for use on the steer. Due to the steer tyres higher load, the tyre would run too hot. Steer tyres carry 5 tonne load, each drive tyre of 8 in the set, around two thirds that. Deep tread drive tyres normally have a fairly aggressive tread pattern, to give them more ‘bite”. The more aggressive the pattern, the faster it wears, so we are almost back to square one. The casing has to be slightly smaller to keep within tyre design standards, so when retreaded, they have to be matched with similar tyres.