Consider this: If your camera had twice as much buffer memory it would be able to take roughly twice as many photos before running out of memory, but it would also then take roughly twice as long for the buffer to clear. So after the initial extra 5-10 shots twice a deeper buffer would allow, you would then still be limited to the exact same frame rate you currently experience until the buffer clears. This may or may not have a significant impact on what it is you wish to capture at high frame rates.

The absolute limit of how many photos you can take over an extended period is governed by the slowest link in the chain. This could be processor speed (which can be dealt with somewhat by disabling certain in-camera processing options), the maximum transfer speed the camera is capable of in moving data to the card, or the maximum speed the card itself is capable of writing. There is also some evidence that camera makers may limit the maximum number of frames allowed in the buffer to match the performance of the processor and write speed of the available memory cards. Three years after the initial release of the Canon 7D, a firmware update allowed the number of RAW files shot at 8fps to increase from about 15 to 25 frames. The number of RAW+jpeg images moved from 6 to 17. The number of jpeg images increased slightly, from 126 to 130 if using an UDMA CF card. The performance improvements allowed all 7D bodies already produced to increase frame rate without any hardware modification to the camera itself, but did require the use of faster memory cards than were available when the 7D was released. It also incorporated improved processing algorithms used by the in-camera image processor, but this improvement was relatively minor compared to the improvement in write speed to the CF card (as evidenced by the dramatic improvement in RAW performance compared to the very modest improvement in JPEG performance).

As has been mentioned on this forum regarding many other features of various DSLR bodies from the major manufacturers, it seems they often limit the capability of the lower end models so that the premium models can command a higher price. What usually happens is that the R&D spent on developing the Pro bodies eventually filters down to the consumer level in the next generation of product cycles. Notice that the 7D firmware version 2 wasn't released until after the 1D X had replaced the 1D mkIV and the 1Ds mkIII. Otherwise, the mid-tiered 7D would have been able to outperform the flagship 1-series at several points relating to burst performance.