Very few players are in Space Age Venus. That is probably the reason.
A lack of players in SAJM should be the reason. Hoods form from the top-down. If it were solely a lack of venus players, that *should* make a split venus-asteroid hood.
I'm in SAV but half of my neighbors are in SAJM or SAT! I thought neighborhoods were supposed to be in the same age.
Most likely you're in a SAJM-SAV split-hood where some of the players recently moved from SAJM to SAT.
It's a numbers game
You need (roughly) 80 players for a neighborhood. Doesn't matter if you have 1,000 players in the Age. 1,000 / 80 would be 12.5 hoods, which means you'd have 12 same age hoods and one split hood with multiple ages depending on how many others can fill that hood from the closest Age/Era
Hoods have a flexible size range. I believe the smallest hood allowed to form at biweekly hood creation is 75 (from new worlds with top hood spanning from the most advanced player down many many ages - those should obviously start at the minimum each week). While the target might be 80. In your example, 1000/80 = 12.5, rounds to 13. 1000/13 = 77. No need to create a split-hood, rather they just make a bunch of hoods of ~77 people. Note this is not a precise example - I don't know the exact target hood sizes for sure (might be 85 as I've seen some that big before) or if they ever fudge them slightly bigger on creation rather than make the split-hoods (they do add people to existing hoods who were inactive at time of hood creation and that can cause them to grow).
Where this does fall apart is when you're dealing with say 300ish or less people. Let's take 200. 200/80 = 2.5, need 3 hoods. 200/3 = 67. That's too small for a hood, so you do 3 hoods and one of the 3 pulls up from the age(s) below to meet the target minimum.