Tires leak from the inside as that's where all the air prussure is. Reasonable sized cracks or weather checks on the outside of a tire has very little to do with actual tire failures. Tire sellers push the even tiny cracks issue to get you to think it's going to BLOW any second to get you to buy new ones! Scare you beyond the reality of the actual situation! They don't sell, they don't eat! Simple economics 101!
The rubber coating inside of the tire backed by the cord/belt structure is the bladder and what keeps the air inside as long as the bead seals properly. Ever notice that when an 18 wheeler throws it's outside tread cap, seldom does the tire instantly go BOOM! Pieces of OTR truck tire are all over our hughways. However, continuing to ride in the tire cords/belts wears them away quickly and then you get to hear the BOOM!
Damaged or broken tire cords are another story and do mean the tire must be replaced and very soon.
Years ago, tires all had tubes inside and all the actual tire itself did was to create support for the inflated tube inside and on the road on the outside.