The problem is this line of reasoning doesn't generally hold up when applied to C codebases (because in practice statistics catches up with you eventually) and C code generators, in effect, happen to contain an admittedly small but rather obfuscated C codebase.

Moreover, you must always include such checks - manually crashing on e.g. null pointers even if you'd be entirely satisfied with a segfault - just to avoid the optimizer potentially going apeshit on your code. Which can have... poor effects on program performance.