Because many of the sales on those platforms currently are classed as “import by private consumer”, directly between seller and buyer, the platform doesn’t get involved.

At least Amazon and Aliexpress should only have to flip a switch, both have EU warehouses at which point the seller does the customs stuff and the buyer is not an importer, any more.

The whole “private import” scheme was never meant for these kinds of trade volumes, anyway, border authorities have no chance to keep up and when they flag down your package it’s a huge pain in the ass because you’ll have to fetch the package, in person, from one of their offices which might be three towns over. And explain to the official that it wasn’t you who mislabelled the contents, or requested to do so.