Pretty sure most memory test tools like memtest86 write the memory and then read it back shortly thereafter in relatively small blocks. This makes the window for errors to be introduced dramatically smaller. Most memory in a computer is not being continually rewritten under normal use.

If you manage to replicate bitflips every few days your RAM is broken.

It's the "once every other year" type of bitflip that's the problem. The proverbial "cosmic ray" hitting your DRAM and flipping a bit. That will be caught by ECC but it'll most likely remain a total mystery if it causes your non-ECC hardware to crash.