It has less to do with “travel companies” than it does with airlines. There are more direct international flights landing in the airports you mentioned than there are into Houston. I’m sure it couldn’t be hard to arrange the tours you want from Houston, you’d just have to change planes before you left the country. You could also book the package you want and just add the cheapest flight that will make it in time to make your connection from Houston. Either way, the added cost of airfare and transfer will have to occur until the airlines start flying more direct flights out of/in to Houston.

Well since Continental and Southwest have major hubs here, I always check their sites first for the best deals in airfare. Then I buy the actual hotel/vacation package separately- this ends up being cheaper and you get more of a choice on where to stay.
As far as packaged travel deals with hotel and airfare, you are right, usually they are from LAX or JFK and you have to pay extra for departing out of houston. If you really want the total package (airfare and hotel), I have had better luck using the continental website or orbitz.

There are direct nonstop flights to Paris and London (continental), but I don’t think there are non stop flights to Spain or Italy.

Tell us where your destination will be ???