Resistance to disease can occur at a genetic level – just like some humans are more resistant to certain diseases. Whereas insect resistance is much trickier. We can select for things like thick, leathery foliage, especially on roses, but ultimately, it’s impossible to say whether they are truly insect resistant or just less preferred if there are other choices. I think that Japanese beetles love roses so much that if they are active in your area and no roses with thinner foliage were available, they’d still feed on the thicker leafed one. So the confidence level in insect resistance is much harder to achieve than disease resistance.