Answer is simple, buy an external disk cheap, take it out of the case and use it as an internal hard drive, you have lost the warranty and apparently for the manufacturers it is a good business...

Unfortunately, I can't tell you why that is. For some reason, however, external hard drives are usually cheaper than loose hard drives. Perhaps it has to do with the predefined environment or area of application of the product, which can be more versatile with a loose hard drive and possibly has an effect on the warranty and its service. That would be my only guess.