We have 50 years of experience of code telling us that, no, programmers are not consistently capable of avoiding memory safety just by being good about it. Saying that it's just a failing of lesser programmers is the height of extreme arrogance, since I guarantee you that you've written memory safety vulnerabilities if you've written any significant amount of C code.
The problem is not that the rules are hard to follow--they're actually quite easy! The problem is that, to follow the rules, you need to set up certain invariants, and in any appreciably large codebase, remembering all of the invariants is challenging. As an example, here's a recent memory safety vulnerability I accidentally created:
int map_value(map_t *map, void *key) {
// This returns a pointer to the internals of map, so it's invalidated
// any time map is changed. Therefore, don't change the map while this
// value is live.
int *value = add_to_map(map, key, default_value());
// ... 300 lines of code later...
// oops, need to recurse, this invalidated value...
int inner = map_value(value, f(key));
// ... 300 lines of code later...
// Hi, this is now a use-after-free!
*value = 5;
return value;
}