How much would a titanium frame cost? Stronger than steel, and lighter, too. I know, I know, the cost/benefit ratio is off the scale, I can hear it now.
Why are things built the way they are? One reason is technical enertia, meaning, that we build upon concepts of the past. Everyone from engineers to suppliers are geared for the status quo, common materials and construction techniques.
In the early stages of horseless carriages, a frame was built by taking two timbers of the most plentiful and easily worked construction material around, or wood. Given the materials and fabrication techniques available of the day, that was perfectly acceptable. For that era! From that point on, we have still been making frames by taking two basic rails of some material, then tacking on all the components. The separation between the components and the framework adds unnescessary weight, but integration reduces weight. Traditional steel frame rails is a very rudimentary construction concept, going back to hewing out rails from a fallen tree, and laying them down to form a basic framework. This principle is as obsolete as a log cabin, since composites and bonding techniques have eclipsed steel frameworks.
If you start with the concept that a frame exists to hang components on, you relegate yourself to the same formulas and design elements that have faced engineers for 100 years. ie, a steel frame, with plastic and aluminum body parts, and various bits bolted to that frame. Sports cars end up weighing about 3000-3500 lbs with traditional techniques and materials.
What if you started with the concept of a stressed skin, where the body itself is responsible for some of the torsional and beaming forces, in conjunction with a composite frame? The body and frame can be the same material, formed at the same time. In essence, there is no separation in purpose and material between the body and the frame. For that matter, you could form the cylinder bore holes and main bearing bosses right into the composite structure! (Provided you had enough strength, or maybe with liners) For many years now, unitized body structure already takes advantage of integration between body and frame, but mostly in steel bodied cars, not so much with RTM or fiberglass, as in the Viper and Vette. In a fully integrated design, each component of the car would lend itself to the strucural integrity of the vehicle, "carrying their own weight". Kind of like the designs seen in nature, such as a simple leaf, where the center spine and leaf surface are intertwined into one seamless unit, and while very weak for our purposes, it boasts an impressive strength-to-weight ratio. The same principles of design can be applied to car design, on a bigger scale, with tougher materials. This is a more advanced design concept, that goes beyond taking two rails with cross members, and hanging everything on that. Oddly enough, the old VW Beetle had an well integrated frame, where the engine/tranny supports were an extension of the center spine, and the front torsion tubes also held the front suspension to the center spine. That is an early example of a high level of component/frame integration. While it was only done on a cheap car, the same principles of integration (albeit of different designs) can work equally well for a sports car, by reducing weight, and providing a high degree of rigidity.
Because of "technical enertia", costs for the latest and greatest technology are prohibitive, so you will see many more common designs of steel rails and X members for years to come. But as newer materials find their way into more widespread use, we may also see more use of stressed skins and composites, as the costs are spread into the mainstream of the auto industry. (Time to short steel stocks!)
Look at the Stealth fighter aircraft, thats where technology is headed, composites, high strength bonding techniques, more integration in design. Someday, the automotive industry may go that way also, and those left behind will complain about change, and be left in the dust of history like wooden wheelsmiths at the advent of the stamped steel wheel. Those who look ahead make the future happen, they invent and create, but unimaginative types who merely cling to the present are soon left in the past.
