Why doesn't Kotlin allow to use lateinit with primitive types?

For (non-nullable) object types, Kotlin uses the null value to mark that a lateinit property has not been initialized and to throw the appropriate exception when the property is accessed.

For primitive types, there is no such value, so there is no way to mark a property as non-initialized and to provide the diagnostics that lateinit needs to provide. (We could try to use a separate marker of some kind, but that marker would not be updated when initializing the field through reflection, which is a major use case of lateinit).

Therefore, lateinit is supported for properties of object types only.

A short answer is that with primitives you can always use 0 as the default, and with nullable types null as a default. Only non-nullable non-primitive types may need lateinit to work around the type safety system.

Actually, there is no need for initializing a variable in Kotlin as long as it has a value before the first access and it can be statically proved. Which means this code is perfectly valid:

fun main(args: Array<String>) {
    var x: Int
    val y: Double

    x = 0
    y = x + 0.1

    println("$x, $y") 
}

But there are (rare) cases when the initialisation cannot be statically proved. The most common case is a class field which uses any form of dependency injection:

class Window {
    @Inject lateinit parent: Parent
}