Armies: Why don't we stick with just 1 army?

Now when I say this it is not to the hard core gamers or the people that just paint for the fun and might end up with the odd game every now and then. I'm talking to most of us that are in the middle.
 Why don't we stick with our armies all the way through?

First, lets compare it rugby teams. You stick with your team right through to the end generaly whether they do well or are in the bottom end of the league. Now lets take a look at armies we hop about with them all the time. Now I'm not saying this is a bad thing just look at the comparison, there are not that many people that will play say imperial fists and nothing else for longer than a few years. Sure they migfht be their favourite but there will always be new stuff that screams to be collected. In fantasy books change in effectivness quite quickly with new releases. So the top tounementeirs have to switch about but why should the rest of us.

Lets get into the meat of this post. Why we do it. It's because of how much of our lives it takes up, we have to change it up now and then I thin k this is good. I like both fantsy and 40k and thing every player should play at least 1 or 2 games of the opposite game of their choice. More than anything else you might l;earn new tactics or tricks to use in your favourite system.

So, just think. About it. I'd love to hear your reasoning behind why we jump about and also from anybody that does stick with their army or anything really. Thanks for reading.

Jacob.