Sycamore is a weed. It's a prolific spreader which is why it's everywhere. Most local authorities are actively removing sycamore from broadleaf woodlands primarily because it's not a native hardwood (it was introduced by the Romans) but also to give other slower growing native species a chance.
Global warming might also be contributing to an abundance of saplings, like they know in a woody sort of way that things are going to get tough, so they're spreading their genes as far an wide as they can so some may survive when times get hard.
Eric