1) Landfills leach toxic chemicals into the water table.

2) Landfills are on scarce land which will run out oneday, making waste disposal more expensive.

3) It can be inconvenient to build over a landfill because you might hit a bale of plastic when drilling holes for your piles.

4) Plastic bags can blow away from landfills and end up in the sea.

5) Landfills may eventually erode into the sea.

1) Isn't true of all plastics or all landfills but nobody wants to encourage the use of the harmless ones. 2) and 3) are such small problems, it doesn't make sense for the whole world to pay whatever it costs to slow them (not even stop them). We'll never run out of land, just prime land in existing cities which is already mostly "lost" to development anyway. 4) is easily solved or a non-problem and probably even worse for recycling. 5) is just a response to a recent news event.

Nobody wants to consider if recycling or banning products costs more than the purely financial future landfill cost problem. I've tried asking someone about the relative costs when they gave me reason 2) and they just responded by saying that their conscience feels better knowing they're doing what's right.

Some of their ideas to solve it cause increased greenhouse gas emissions, like you mentioned with the energy cost. Many people even want to burn it!