Assuming you wash your other produce (you probably should, especially if you buy organic and don't like e. coli.), the question you're asking is just "how does the crap on the outside get into the inside?" The usual route for those contaminants is peel -> hands -> fleshy fruity bits. A lot more is transferred than you might think.

For a bit of fun, try the same idea with a few dirty potatoes. Wash two thoroughly, and leave one unwashed. Peel a washed one, peel the unwashed one, and peel the last washed one. You'll observe that both of the latter two are much dirtier than the first one [0]. It won't be as dirty as what you started with, but it'll still be enough to cause some minor taste issues and (depending on the type of soil) make the dish feel gritty as you eat it. Contaminants you can't physically see/taste/feel are transferred the same way.

[0] There's a minor confounder in that potato flesh can oxidize, and that discoloration might be perceived as dirt. It's not as bad as with, e.g., apples though. Just work quickly, and the difference in dirt levels should be apparent.

Once I saw how fruit is transported and stocked, I wash all of it regardless of whether I am going to eat the skin or not.

Maybe in order not to get the (particularly bad, as far as I know) pesticides on your hands? I wash my hands after having peeled an orange, but washing the orange itself seems like a waste to me.

I don't really know. I could understand if it was for zesting, but then you're supposed to use untreated oranges anyway because the pesticides don't wash out easily because of the oils on the orange's skin.

She is 1.9 years old and anything that looks like food will be bitten. But to tell the truth, you even need to wash bananas: pesticide residues (you can transfer chemicals to your hands and then to the editable part), can collect dirt/bacteria/rodent excrement.

The skin might be dirty, so cutting a knife through it will slightly contaminate the insides

I’ve read online that the skin of fruits can contain bugs and larvae that when washed help keep your fruit longer and prevent things like fruit flies.

i came here to leave the same comment, and I checked beforehand if someone noticed the same.