Because there's a lot more involved in performance than the raw port speed. A consumer router designed in the 2007 time frame was shooting more for the 50mbps you're seeing. 100mbps is an ultimate limit but not the costly one of processing power.
If you're interested, I benchmarked similar hardware in 2008: »
Benchmarking WRT Firmware... Some Surprises!The winner was Tomato firmware at 58mbps.
Notice how even when it's LAN-to-LAN 85mbps was as good as I got. The penalty is because LAN-to-Internet requires more processing power.
EDIT: And sorry if I sounded like 100mbps ports was the ONLY reason. If you could magically change those ports to GigE you wouldn't see much gain.