Here is the major difference.
Juve already had the entire infrastructure in place. The roadways, the footprint of the stadium, the land, everything. Wht Juve basically did was remade and restructured the Delle Alpi for this new stadium. That is why the costs are not nearly has high to build a new staidum as it would normally be if they had to acquire land, build additional routes of transportation, and build an entire new stadium.
This is something that both Milan and Inter would have to do if they wanted to acquire their own stadiums. i highly doubt that the city of Milan would allow the San Siro to be torn down in order to have a new stadium put in there. Its an old stadium, for sure, but it is not antiquted. There have always been updates made to the San Siro. Its still a very viable all prupose facility.
The only Solution that Milan and Inter could come up with is if they got together and built an all new stadium that they would have to share. That would split the costs 50-50. However, to build all new grounds, to build an 80,000 seat state of the art stadium for each club could very well cost them 1 billion dollars each to do so. Sharing a new stadium would be the only way to defray the costs, and it would also cut their potential revenue in half.
Let's say that the city of Milan does indeed allow the San Siro to be resturctured. Where would Inter and Milan play? As far as I know, there are no other football stadiums in the city of milan. That is another advantage that Juve had, being able to use the Stadio Olimpico.