Oh, but it does.
With Luke, for example, the father you get along the timeline, the stronger he gets. There's no point in his life where he was better last year then he is this year.
Mara's the same way. She gets better every year that goes by. By Sacrifice, she's at her peak.
And yes, Caedus was injured when Mara finally crossed blades with him. The difference is, Mara caused all those injuries herself. Full credit for having sense enough to weaken your badass opponent before going head to head.
Also, Anakin only wins on those flat, featureless planes. Put Anakin vs. Mara on, say, Mustafar...and the poor boy's going to Obi Wanned all over again.
It's based on the idea that if life creates the Force (see, Yoda, ESB, and if anyone knows what he's talking about, it's Yoda...)...there's more living beings alive now then there was in say, the TOR Era.
Consider the exponential population growth in real life, now multiply it by all the planets there are, and you get an idea of why the Force itself might be growing stronger over time.