>Because integration tests come with (extremely) high costs.
Unit tests usually have lower capex and higher opex. It often takes less time and effort to write a single lower level unit test but that test will require more frequent maintenance as the code around it evolves due to refactoring.
Integration often tests have higher capex because they rely upon a few complex integration points - e.g. to set up a test to talk to a faux message queue takes time. Getting playwright set up takes quite a chunk of up front time. Building an integration with a faux SMTP endpoint takes time. What is different is that these tools are a lot more generic so it's easier to stand on the shoulders of others and they are more reusable and it's easier to leverage past integrations to write future scenarios. E.g. you don't have to write your own playwright somebody already did that and once you have playwright integrated into your framework any web-related steps on future scenarios suddenly become much easier to write.
Whereas with unit tests the reusability of code and fixtures written in previous tests is generally not as high.
You have to also take into account the % of false negatives and false positives.
I find unit tests often raise more false positives because ordinary legitimate refactoring that introduced no bugs is more likely to break them. This reduces the payoff because you will have more ongoing test failures requiring investigation and maintenance work to mitigate this.
I also find that the % of false negatives is lower. This is harder to appreciate because you wouldn't ever expect, for instance, a unit test to catch that somebody tweaked some CSS that broke a screen or broke email compatibility with outlook, but these are still bugs and they are bugs that integration tests at a high level can catch with appropriate tooling but unit tests will never, ever, ever catch.
>But when you skew the testing pyramid, or worse, make it an testing-ice-cream-cone, that ROI can and will often quickly become negative.
The pyramid is an arbitrary shape that assumes a one size fits all approach works for all software. I think it is one of the worst ideas to ever grace the testing community. What was particularly bad was Google's idea that flakiness should be avoided by avoiding writing tests and applying good engineering practices to root out the flakiness. It was an open advertisement that they were being hampered by their own engineering capabilities.
I do agree that this is a cost/benefit calculation and if you shift some variable (e.g. E2E test tooling is super flaky and you've got good, stable abstractions to write your unit tests against, you've got a lot of complex calculations in your code), then that changes the test level payoff matrix, but I find that the costs and benefits work out pretty consistently to favor integration tests these days.