"Properly constructed and operated landfills" which prevent leakage of microplastics and toxic decomposition products into air and waterways are far from being cheap, especially on longer time frames and with involved volumes (more than 80M metric tonnes per year just in US!). Usually, you either end up with slack maintenance, resulting in eventual pollution of the environment, or you export this waste into third world countries with significant amount of such "exports" ending in oceans.

And plastic does not stay inert in landfills forever either, it slowly decomposes, releasing products into environment (methane and carbon dioxide being most benign of those). Effectively, you get slower, less controlled and more dangerous (biologically speaking) "burning" which will be done for many generations instead of burning the stuff right away in controlled conditions.