Agreed, also i like to stack my bag very methodically with heavier and more sturdy items at the bottom, cold things together, interlock and tetris everything so the bag has its own integrity and doesnt collapse and spill items out into the car after a corner.
Yes i know the checkout person also stacks in a similar fashion but they are paid the slap it through and into a bag as fast as possible to serve the next customer and i dont mind sacrificing an extra 15seconds of my day over the bag stacking operation to do it right.

Not to mention that I prefer using my own bag(s) when I can, and whenever I go up to a manned checkout especially with a bagger and hand them my bag to use, at least half the time they start to panic as if they haven’t seen this before, and then they’re eager to start using the plastic bags after my bag is barely half full. This is even in stores that sell their own reusable checkout bags. At the self checkout, I can use my own bag worry-free and fill it however full I want, and if I overfill it, that’s my problem and not on the store, so they don’t have to worry either.