Well, not all mobile systems are created equal. For example, standard mobile systems (i.e. office systems) do not require to have really powerful components as they just simply don't need them. The components on these systems require less battery power and can even clock down the processor and turn on/off devices to consume less power and extend battery life for 2+ hours. This happens especially on office applications as the system simply does not need its full power to run the application.
Systems with higher powered components (gaming systems, 3D working stations) have higher power requirements that demand more power from a battery, keep in mind that you're having a dedicated graphics card, faster motherboards and depending on the system itself, larger screens and even multiple hard drives and optical units. Also, these systems run more powerful applications which do not allow it to clock down or turn on/devices off and on as it would need to extend battery life.
Therefore, the battery life of a mobile system with these high powered components is reduced compared to less powerful systems.
The increasing power of today's mobile components on mobile systems are just simply demanding more power from the batteries resulting in more reduced battery life than those provided on older mobile systems that did not have as powerful components as today's systems are.
Making a battery is not simply a matter of just "making it". Depending on the size of a system there is a limited amount of cells it can physically hold (or require for powering up the system). Companies do think about their customers but there is a limit to what can be done physically and electronically.
Example: Why couldn't we just be able cram a Ferrari engine into a Beetle? After all you should be able to because they're just cars right?
Different customers have different needs, to some, they just need a mobile system to do office work while others need lots of power but are unable to take a whole desktop system with them.
Whichever you need, systems are made regarding the technical requirements of the internal devices and components inside a system and the space things are being put into. Again, it's just not a matter of simply "making it", things have physical and technical requirements to work properly and, most of all, safely.