For the same reason there has always been a separate career, for those nobles that participate in government. Those that don't, simply have the title, and lands, heck, maybe even a proxy vote, and enjoy being wealthy. Perhaps they are off adventuring, or does a person lose their title once they stop the career? Saying that you have to be in that career in order to have the title kind of goes against things all the way back to CT, when there was no separate noble career, but PCs could still get the rank. And yes, I'm willing to accept that perhaps things have been changed for T5 since many things have (for better or worse), but if so, I don't think it has been made clear. It really wouldn't take more than a sentence or two from Marc to clear up the whole thing, one way or the other.
That's fine, and I can see where you are coming from on that given that 100% of the people you are playing with 'get it' the same way you do. But, you have to admit it is a rather small sample size, statistically speaking, and could very well be not representative of the greater Traveller playing population. So yes, I can certainly see why you think this way, and feel supported in that view, that's fine. But for those (at least some) of us out here, it doesn't help support the actual case.