Except single use plastics don't end up in the ground reliably: they end up in water ways, beaches, wetlands and the oceans. They degrade into micro plastics and then end up in the food chain.
They are a huge problem for non-emissions reasons.
Oh yeah, they're definitely a problem, and work should be done to reduce and eliminate them. But they're not directly a climate change problem.
Also, if you live in a major western city, especially a non-coastal one, your garbage is pretty reliably ending up in a landfill. 90% of river-borne plastic pollution comes from just 10 rivers, all of which are in Asia and Africa: https://www.weforum.org/agenda/2018/06/90-of-plastic-polluti...
This is not a get-of-jail-free card to the rest of the world, but it does change the nature of our responsibility— in particular, it means that individual action is almost entirely worthless. Because the issue in the west is not the trash, it's that we're culturally exporting a single-use lifestyle that creates all that mess when it is imitated in places without the pipeline in place to manage the waste. So what is needed is a collective cultural change in order to alter what we export via our media, and the actions/expectations of our politicians, celebrities, tourists, etc.
They are a huge problem for non-emissions reasons.