Specifically for PLA, there's the issue of methane I linked in my reply to your other comment. For other plastics, the problem I see with landfilling is mostly that you spent a lot of energy creating this material, and now you're throwing it away and spending that energy again to create more. On top of just being wasteful, and emitting a bunch of carbon, all that material production is also funding an industry that is hellbent on destroying the planet.