Because with the 128GB, you're probably left with around 100 to 110GB after Mac OS X installation and updates etc. Then it is good practice to leave at least 10 to 20 percent of the SSD free for virtual memory etc, therefore about of that 100GB you will need to leave at least 10 to 20GB free therefore only giving you 80 to 90 gigabytes of usable space. Now this is without any applications etc.
In my opinion, and not to sound harsh, if you can afford to buy a rMBP, then you can afford the extra £200 to pay for the extra storage. If money is really that tight for you, then you should be looking at cheaper options and not shooting yourself in the foot by limiting your storage.
Now, with mobiles it's slightly different, people upgrade a lot quicker and therefore can live with the limitations of low space. Normally people keep Laptops for a lot longer, I for one keep them for at least 3 years (duration of AppleCare plan)