I've always assumed that this is mainly down to lack of upper body strength to "pull" you though the water. My legs don't do a huge amount (see kicking comment!) but I figured that as a tri aflete then we want to save legs for bike/run this isn't all bad (seeing as I'm never going to win anything other than last or a 1 pixie race!).
When I did my 5k swimathon last year my splits weren't much different from start to end - same with my running, never fast just keep plodding.
The run plodding (and slow bike) I've put down to the excess lard as that must slow me loads