Why is hard to write secure code in memory-unsafe languages like C?

I understand that random unexperienced coder will not write high quality code but what I mean are professional teams of coders with very well defined rules with various code-check tools like in Google, Microsoft etc. How come that this class of flaws are so common? The fix of the flaw is not like 10 kLOC - usually it's quite simple. How come there are no automated tools that can scan the codebase and find such issues beforehand?

check class code coder coders cybersecurity defined etc fix flaws google hard high languages memory microsoft professional quality random rules secure code teams tools understand