Why do they say that Internal Hemorrhoids usually don't hurt unless they prolapse to the outside, when mine are inside the anal cavity and hurt like h?

Usually. Usually means usually, unfortunately, you appear to be the one that they do hurt in. Talk to your Dr, as if the pain is that bad, you can get medications and treatment, even if surgically, to remove and cure the internal hemorrhoids. Good luck to you.
Nerve innervation. External hemrrhoids can originate just on the inside brim of anus. A true internal hemrrhoid doesn't have the same nerve innervation as the distal part of anus. As such, it is less likely to hurt. If they get large enough, you can experience pain when it comes in contact with and irritates the mucus of lower half.