Because it's been tried before and for the most part it was an abysmal failure. I was just starting out doing some basic web freelancing as a teenager in the 2000s and even I got roped in to clean up an outsourced project after being outbid a year earlier by an overseas firm during the first outsourcing wave. Lots of people on HN have horror stories of cleaning up from that era.
We've been here several times before - like literally just this past year of everyone saying "oh but now you have to compete with remote workers everywhere!" Salaries keep rising because software is an arms race. The companies making the most profit will continue to invest in getting the best people and outside of the odd global crisis, the industry will continue to grow as everyone else tries to keep up both in technology and in hiring. All those firms I cleaned up after as a kid are still around today and bigger than ever, yet on this side of the ocean we keep making more and more money.
I think we've got at least a century before software hits the diminishing returns that the industrial revolution did. My local lumberyard is still using DOS machines probably made before I was born.