Why are some aircraft built in a single country, whereas others are a collaboration of many countries?
Why did Eurofighter need collaboration among four countries?


For production volume.

We are still in an era of relatively low-volume high-value combat aircraft compared to the early years (say pre 1950).

Cost per unit is less if the development costs are spread over a greater number of manufactured units.

Individual European nations each have requirements for relatively small numbers of aircraft. If they each develop and manufacture their own aircraft, costs to each are likely to be higher.

Politically, it is easier to sell a military aircraft to politicians whose countries benefit from the manufacturing.

There are also benefits in sharing development costs.

Given the close economic, political and military ties between European countries, collaboration makes sense.