Greenhouse effect. Even if you have the most expensive tint or roof insulation, it will only buy you several minutes before the heat starts building up.

For example, if you have two identical cars with one treated with good tints and insulation, it will take a bit more time to warm up the interior of the treated car. This is effective only if you leave it in the sun for a few minutes. But if you leave both cars bathing in the sun for an hour or the whole day, both cars would be equally hot inside by the time you get to it. And ironically, the treated car would retain the cabin heat much longer than the untreated car long after the sun has set.

Tints and insulation will only slow down the heat transfer, not stop it. As many have mentioned, it can't replace a covered car park or garage.