Because there are a lot of other factors that are different. Other countries have different interest rates, tend to have much smaller markets to sell into, and tend to have much more tightly regulated labor markets. There's also an absence of large tech acquirers that exist in the US.
VC economics rely on a portfolio theory of a few gigantic winners making up for the other losses. So if different economic conditions creates a difference in the ultimate size of our 'winners' than you have to figure out how to have fewer (or smaller) losses.
Your points are true, but I think other factors also play a role. What makes, for example, Australian investors behave differently to American investors? No doubt differences in economy size, regulatory regimes, etc, make a difference. But I also think some of it is due to differences in national culture. People have been talking about "tall poppy syndrome" as an element of Australian culture for decades; people don't say the same thing about American culture. (That said, it looks to me like the concept, but not the name, is increasingly making inroads into the US.)
Certainly investor culture matters a ton. And US is a big place and certainly isn't monolithic. All the stories of early silicon valley speak of a clash with 'East Coast' investors who are viewed as being more conservative on both terms and stages.
It's a little goofy, but I have a personal hunch that the high level of sunlight in Silicon Valley help entrepreneurs there - as building companies is rough so it helps to persevere if it's always bright out.
(Background, I grew up in Boston, moved to Silicon Valley, and have tried to start companies with varying amounts of success.)
VC economics rely on a portfolio theory of a few gigantic winners making up for the other losses. So if different economic conditions creates a difference in the ultimate size of our 'winners' than you have to figure out how to have fewer (or smaller) losses.