My guess would be that it is because each country has its own rules about how VAT works for for things sold to buyers in their country. Each country would like to fully apply its rules to all purchases by its residents, but they recognize that it would be unwieldy and expensive for sellers to have to deal with minutia of the VAT rules of a couple dozen different countries.
VAT MOSS is a compromise that simplifies the rules, so that a seller selling into several foreign EU countries only has to deal with one unified set of rules through the VAT system. Instead of having to know details of the tax law of several countries, the seller only need to know the VAT rate for each. That's much easier to deal with.
I'd guess that they don't apply VAT MOSS to sales by domestic sellers to domestic buyers because they consider their own tax rules superior to the rules under VAT MOSS.
My guess would be that it is because each country has its own rules about how VAT works for for things sold to buyers in their country. Each country would like to fully apply its rules to all purchases by its residents, but they recognize that it would be unwieldy and expensive for sellers to have to deal with minutia of the VAT rules of a couple dozen different countries.
VAT MOSS is a compromise that simplifies the rules, so that a seller selling into several foreign EU countries only has to deal with one unified set of rules through the VAT system. Instead of having to know details of the tax law of several countries, the seller only need to know the VAT rate for each. That's much easier to deal with.
I'd guess that they don't apply VAT MOSS to sales by domestic sellers to domestic buyers because they consider their own tax rules superior to the rules under VAT MOSS.