Use tools to make it easier. Write test suites before you write the code they test. All the resources that you used to put into static type checking, put into testing instead. Smalltalkers claim that you get fewer bugs over-all if you take this route. Conventional static type checking takes big effort and delivers small gain, especially on top of Smalltalk-like testing.
Some people believe programming is so hard that almost any gain is worth the effort.
Other people believe the available effort is limited and want to maximize the value before they run out. Or to put it another way, their process and effort leads to commercial-level quality and further effort isn't worth it.