One thing to consider is that most code that gets written is not built to be highly testable. This makes testing harder to the point that "it's not worth it". If you optimize your coding style for testability, writing tests is faster and easier.

The problem is, writing testable code requires more thought up front not just about individual objects and methods, but also the overall architecture of your project. If your code is highly tied to an ORM or framework, then it's going to suck to test.

Also, not every language is equally easy to test. For example, Ruby has an obsessive culture around testing, so they have a lot of fantastic testing tools and the language itself makes things like mocking pretty easy. PHP by comparison doesn't value testing as much as Ruby does. The testing tools are worse and there are fewer of them. Also, due to the lack of monkey patching, you have to write your PHP code with dependency injection and testability in mind from day one for it to be highly testable.

In short, testing is a useful thing, but most code isn't written to support it and many languages and frameworks and patterns encourage you to write unpleasant to test code. If code is hard to test, programmers won't write tests unless their job requires it.