BoxRec

Athletes in sports like football, rugby and tennis no longer do long runs as part of their training regime like they used to. They now incorporate stamina and endurance into more technical elements of their training.

With that in mind, why do boxers still do 6 mile runs before sparring and gym work?

Quite often in the build up to fights, I hear boxers talking about how much running they've done and how fit they are but during the fight they burn out quickly.

Is more intense sparring better for boxing endurance and fitness rather than long runs?

Boxers don't do long runs for fitness, long runs are for weight maintenance.
Sprints and intervals are for fitness.

You can do longer runs with intervals if you don't sprint. Three minutes at 80-90% max heartrate, a minute of recovery jogging, and then repeat until you're knackered. I'm personally a big advocate of running ahead of 'funny drills to Rocky music' in gyms.* Skills are largely innate, whereas fitness is an ongoing, vital pursuit.

*There's a widespread belief that exercises are automatically tough and effective with a big enough number of reps. So long as you can do a series of reps, counting up to high numbers to boost your ego's silly. But so many guys work to fatigue on their first set.