Why are there no weight-based shipping charges for international shipping as there are for shipping within Germany?

Spare parts will be shipped by us as an insured package with tracking, customs clearance, risk surcharge, Brexit costs, tolls, etc.. As a contractual partner of DHL we have to decide. Shipping either weight based or flat rate. Since the weight-dependent international shipping is already more expensive than the flat rate from 10 kg, we have decided on the flat rate for international shipping. You can find the exact shipping costs for your country in the shipping and delivery conditions. Note: The shipping cost includes 1,50-3,00€ packing cost depending on the size. All other costs are the costs that DHL charges us.