There's a few practical reasons why your home internet is slower than wan. Firstly, the speed you can transmit data with diminishes with distance - the longer the distance, the weaker the signal gets, and the more interference occurs.
With home networking, you're talking of runs of tens of meters (common 100BASE-TX and 1000BASE-TX goes up to 100 meters). You're also serving a single system over this.
With a WAN you're looking a distances in kilometers, with bandwidth shared by many users.
With ADSL you're working with a limited part of the frequency spectrum (though adsl can go from 8 down, 1 up to 24 down 3.3 up) which slows it down.
Cable can go significantly faster, but most ISPs share bandwidth between users in an area.
You can also get gigabit or faster speed on fiberoptic links
At the end of the day though, it costs you maybe 10 dollars for that cat6 cable. Your ISP needs to dig up roads and lay much more expensive cable, and lots of it, so it makes sense for them to spread bandwidth out. That said, if you're willing to pay for it, there's no reason why you can't get lan-like speeds on a home or office internet connection