1) Plastic is not liquid, so you can't pipe it to a gas or oil power plant. You may argue that coal isn't liquid either, but continue reading...
2) Burning plastic generates toxic fumes.
3) Plastic ash is sticky and very difficult to clean.
It's a fascinating topic. There's even more problems than the ones you bring up, but engineers are also pretty smart.
1) Plastic is not liquid, so you can't pipe it to a gas or oil power plant. You may argue that coal isn't liquid either, but continue reading...
2) Burning plastic generates toxic fumes.
3) Plastic ash is sticky and very difficult to clean.