This is a direct result of the instruction post 9/11 that arriving and departing passengers must be physically segregated - it would have required a full demolition and rebuild to deal with in a better way. It is right that 17 years and counting since that change without a rebuild is too long though.
Indeed the new pier 5 at Gatwick's North Terminal was rendered unfit for purpose almost overnight before it opened as arrivals and departures were on the same level, cue BAA's love affair with glass doors. Pier 7 at LHR remains so afflicted, and DUB's 200 gates have had an upper floor added in recent years to mitigate this. Strangley, the older LHR T4 was fine as it had been designed to have arrivals and departures on different levels. Those were different days, smokier, more expensive, a world of cassette tapes and VCRs......