Because the bulk of the work of an MP is in being at home spending time talking to the bosses (i.e. the constituents) to ensure that the correct message is brought back to the central meeting place (i.e. parliament). After all, you, the constituent, are hiring the representative to carry your message to the central meeting place so you don't have to make the trip yourself. But you still have to make your message known. MPs are most definitely not mind readers who can simply be elected and then never spoken to again. It is a time consuming process to sit down with each and every constituent to hear their voice.
And, well, I guess if an MP really is a mind reader and doing such a great job that his bosses don't feel the need to talk during these recess periods, why not allow a nice vacation? An MP figuring out how to be more efficient than other MPs shouldn't be penalized. The recess will still be necessary for the poorly performing MPs to go home and get tuned in by their bosses.