Avocados come from a wet environment. The Guatemalan and Mexican races come from cloud forest environments that are dripping much of the year from if not rain, a high humidity that creates a cloud-like condition. These are conditions that leaf fungi love, so in order to protect from leaf fungi, they have developed a waxy cuticle so that water runs off. This also protects the leaf from water that would dissolve nutrients in the leaf. This is one reason that foliar nutrient sprays don't work well with avocado. They can get into flowers, so floral sprays can work. But then you might mess up pollination.

Another consequence of this cuticle is that the leaves are resistant to decay. One reason for the thick leaf duff/mulch/layer found in avocado orchards.

I tagged leaves that had newly fallen with different colored propylene string at time different times of the year. Leaves were 1 month, 6 months and one year old. The leaf on the right is one year old. That waxy layer is really resistant.