Let's consider the biggest city in the U.S., NYC. Manhattan? An island of 1.8 million no more than 2 miles wide. Brooklyn and Queens are on a spit of land called "Long Island". Together they have 4.8 million residents. Staten Island is, you guessed it, an island. And the Bronx is necessarily very close to a lot of water.
People litter. Cigarette butts, straws, plastic bottles, etc. all very regularly and easily wind up in the ocean with an assist of a slight breeze.
I saw it in action firsthand in NYC. A garbage truck picked up an overflowing trash bin around midnight, and the dome of trash on top fell off and into the street, and the worker did not even break stride. Many well-meaning people took the time to carefully place their trash on the trash that was in a garbage bin, but they may as well have just dropped it on the street directly for all the good it did.
After I saw this happen a few times, I stopped taking the time to carefully place my trash, and just held onto it until I found an emptier receptacle. After all, emptying public trashcans is already a terrible job. The garbageman isn't carrying a broom and dustpan. Why make his job more difficult?
(I would distinguish public trashcans like this from e.g. commercial trashcans for the use of customers of private businesses. First, they should never fill over halfway. Second, the whole thing should be kept clean. Anything else is a failure of management.)
Our weather patterns here include heavy rain storms. What I don't happen to catch gets flushed away into the drain in the street, which leads to a nearby creek (stormwater is kept separate from sewage here).
And of course, all the other parts of the road that don't happen to run up to a yard are not really groomed, so all that debris is flushed into the waterways.
I assume this cleaning includes putting them in your trash and the city taking them to a landfill?
It's obviously not the best path, but cities[0] (by necessity) generally do a good job cleaning up the trash that gets scattered around.
Bigger issue is non-urbanized areas where there isn't the density/wealth to clean up litter.
[0] The collection of people and institutions, not just the government.