Why do you even need to buy SIMs? Are international plans that pricey in the U.S.? Here in Europe they are quite affordable, my Czech 'connect for good for abroad' is $0.35 per MB which is enough in most situations.
The average web page size is now almost 3MB [1]. At that rate, it costs about a dollar to load a single web page and about $350 to download a gig of data. That's not affordable, especially when you can get the same service at a fraction of the cost by buying a local SIM. For instance, it costs $5 for 4GB of data in Cambodia. It would cost you $1,400.
We do have Google Fi, which is $20/mo + $0.01/MB. It works in 180+ countries with no need to swap out SIM cards. The main downside is the lack of phone choice...basically only the Google flagship phones (Pixel and Nexus...not the worst choices) because of something to do with the radio hardware they use.
For the casual traveler, it might not make the most sense, but for someone who does a lot of international travel, it's a great option.
[1] http://httparchive.org/interesting.php