The difference between scratches in glass vs scratches in plastic is... Scratches in plastic 'seal' back up when the edges of the scratch get pressure on them (say from cleaning)... In glass it just stays as a grove and can be sanitized.
It's more than that.
Plastics are built from molecules that lipids can bond to very strongly.
So strongly that removing that bond chemically risks damaging the plastic.
This is the same reason why you can't reliably make a meringue in a plastic bowl.
Lipids - this is to say fats and oils - stick to plastic like magnets. On a large plane your sanitizers and surfectants have a good angle of attack, but in a scratch they can't easily get all the way down in there, and the lipids provide a haven for bacteria.
Now you are going to say, there are no fats or oils in your brew. Well, you're wrong. The amount of fat in your brew is vanishingly small, but it's more than none.
There are oils in the hulls of the barley, and some of that makes it into your brew. More than none. I have a friend with a mass spectrometer and an electron microscope who can prove it.
Not to mention the lipids on your skin, on the surface of your rubber gloves (transferred from your skin), etc. Not much, but more than none.
If you have an autoclave, and the plastic is autoclavable, you can rest assured that straight potassium lauryl sulfate with pressure and heat can do the job. But you don't have an autoclave.
Glass can provide a little valley for the lipids to collect in, but not the ideal bonding area for a lipid the way that a plastic can't avoid providing.