I assume that the available heads differ somehow, but don't understand what those differences might be and if they're functional, personal preference or a bit of both.
Most tripods are sold without a head so that you can decide independently on each part.
They both have to match in terms of capacity but otherwise you are free to choose the right set of legs and heads according to your requirements. If they were bundled together you may not find the right combination for you.
Particularly there are way more variety of heads than legs. They vary in precision, speed, motion control, balance, etc.
Probably for the same reason high end bodies don't come with a lens.
This gives you more flexibility with what head you purchase, and to allow for the professionals that already have one or more heads.
Choice for one size does not fit all. For some situations a 3 way pan/tilt would be fine but for others a ball head is a must.
Manufacturers offer a variety of possibilities to match the conditions under which the head will be used.
At last, it depends on your taste, subject matter and ease of use.
It has possibly been said already in the answers above, but the answer is, different needs, different heads.
Video needs a head that can be moved smoothly for example. Many people like ball heads because they are quick to set up, BUT they are hard to use when shooting for example a panorama, a more "classic" design is better suited to that.
By selling tripods without heads, it is possible to offer maximum flexibility with few products, plus you can mix and match heads and legs between manufacturers as well.
Also, something reasonably expensive that comes in two separately available parts will be preferred by many customers (especially the serious amateur or semi-pro that doesn't want to deal with insuring their gear), when sold at the same price as something that doesn't, because there is less risk of total loss from breakage or wear.