The soft playing surface and the lack of people on the court are just some of the reasons why beach players' have fewer injuries and play for more years. Volleyball in general is an old-person's sport (players reach the top of their game in early-mid 30s...compared to most other sports were 29 is considered old and on the way down (football, basketball, etc).
There's 2 kinds of injuries: recurring and incidental. Incidental injury occurs from a specif act -- like falling & spraining an ankle. Recurring injuries happen over time, -- like years of armswings with poor technique yielding in shoulder pain. Incidental injuries tend to be more severe, whereas recurring injuries will slowly be revealed.
Recurring injuries occur in both beach & indoor. Indoor players do the same thing over and over and over, and beach players touch the ball TONS of times which can also cause recurring injury. I notice Kerri Walsh with the shoulder tape for many years and Clay Stanley without - so indoor/outdoor has nothing to do with recurring injury. Recurring injury is generally caused by technique or muscle over-compensation.
Incidental injuries on the other hand, occur less frequent on the beach. There's less people to bump into: there's
Typical indoor incidental injuries:
twisted ankles (from landing on an players foot),
dislocated fingers (blocking),
knee ligament tears (landing & turning)
Typical outdoor incidental injuries:
dislocated fingers (blocking)
Some of the reasons sand has fewer injuries (which means a longer career):
-The playing surface is soft and has lots of give
-Beach players do all the contacts which means their muscle growth is more balanced overall (vs indoor players that do 2-3 sets of movements for the entire season)
-Doubles strategy doesn't require massive amounts of power to get kills (only 2 people defending court)
-The distance between you and other players is much less. Indoor players have 150 square feet per player while outdoor players have 364 square feet per player. That's 2x as much room.
-B/c there are only 2 players, the only touch between partners that's in doubt is the pass (if i pass, that means you have to set). indoor there are 6 which means any touch between teammates can be contested and therefor cause an incidental injury.