It may also depend on whether there is still a bit of air trapped in the tire. If it it floating vertically instead of flat, trapped air may be the reason.
I have found and removed multiple tires from the bottoms of waterways. They are heavy, especially if they are big truck tires.
But, they are not so much heavier than water, so they can get moved around by currents and waves.
Some decades ago, somebody had a "great idea" about how to get rid of used tires and also "help the environment" by making "reefs" of old tires off beaches on the Atlantic coast. It was true that sea life would attach to the surfaces when the tires were on the bottom.
But, it turned out to also be true that storms could wash those tires to shore. So, the practice was stopped. Now they use things like concrete blocks for synthetic reefs.