The answer is that in some places, for good reasons leaves need to be cleared and other posters have listed these, while in in other places they can be let lie.
What matters is not whether they are gathered or not but what happens once they are gathered up. Most gardeners, if the leaves are not too many compost, them and return them to the soil, where they want them.
Most if not all local councils run or contract in green composting schemes, where they deliver all the leavesand green material they collect, plus the contents of green bins at tips and take them to these industrial composting schemes and then either sell the composted material back to gardeners or use it to improve the soil in council parks and gardens.