This is true, but they also set odds to pull in the most $ they can. Odds aren't always the most logical analysis of everything, they're the most cost-inducive.
At this point just after the season, sure odds are great for laying perhaps a future money bomb down, but essentially right now they're virtual Hail Mary's. There is way, way, way too much stuff that can happen (free agency, injury, act of God, whatever) to give anyone a true idea of how next year turns out. If you take a look at this page, you can see that most of the WS winners (since '85 when this began) were nowhere near the favorites (very few were < than even 300+ odds; in fact most of them were huge underdogs). Conversely, very few of the favorites just after the season end up winning, or in many cases, even being close. You can see the odds of course changing often through the seasons on that page. And a great majority of the time, a team in the WS the year before doesn't make it back the next year.
Setting odds now isn't much of a guarantee of anything, even a non-cellar finish.
Of course the teams that finished strong this year will be near the top now, but that's just to get bettors in on the now-less-focused action (this is where they hope that a guy lays down that "longshot" bet in Vegas or whatever).
The Astros have a strong core and of course, nothing else factoring in right now, should be a contender again. But odds at this time of the season? They don't mean much except for a slap bet in Vegas.