Why are Gatwick's runways too close together?
Historically, Gatwick had only one runway (08R/26L) with a taxiway to the north, but this obviously would cause problems if that runway was out of use for any reason - emergency, scheduled maintenance, etc.

In 1979, the taxiway was upgraded so that it could be used as a runway when the main runway was out of use. Since this used existing infrastructure, the locations were obviously fixed. 

However, at that time the then owners of the airport (BAA) agreed with West Sussex County council that the second runway would only be used when the main runway was unavailable. This was to protect the interests of the airport's neighbours. With this agreement in place there was no incentive to address the issue of separation. The agreement was set to run for forty years, and expired recently on 13th August 2019.

With the imminent expiry of that agreement, the airport is now exploring ways to  bring that second runway into use. One suggestion is to widen the existing main runway southwards, allowing the centerline to be moved south and increase the separation.

Source: The Independent, 15 October 2018