If the humans continue to stay in their base, the game will get really boring. Since Tremulous is a game (in other words, it's pointless if you don't have fun, which you won't in a boring match) as opposed to a real war where people die, it's in everyone's interest if the Humans get out of their base and attack when the Aliens catch on.
That's the point. Some people are so stupid that they don't even play the game for fun , and stick to that lame camping behavior just because it prevents the other team from winning. It is highly frustrating for the aliens , because such a behavior is not counterable - it is even encouraged by the game rules.
Think about it... how are humans rewarded credits ?
1) Kill bonus. Player gets credits for doing the most damage to a killed alien. Which behavior is most successful ?
a) Camp and wait for dretches to be visible , shoot them when they are. Lure them into dying to turrets , get credits for damaging them.
b) Get out of the base and fight aliens yourself (likely dying in a split second from 2 hits from a skilled dretch) , and chase retreating aliens to finish them off - killing credits go to the chaingun/pulse/luci spamming base camper who did the most damage.
2) Survival bonus. Player gets credits for staying alive longer than 2 minutes. What to do ?
a) Camp in a safe place (behind turrets) out of reach of melee aliens , let the credits come in without doing anything (or shoot passing aliens when you're bored)
b) Get out of the base and try to survive separated from most of your teammates , just on your own with your limited supplies. Try to keep aliens at bay with the ammo you have , use your only medikit to heal if you aren't killed in a split second , and run the hell away from 2 hungry dragoons as you're out of ammo.
Options a) are the easiest and safest way of earning credits , but no fun for the alien team. They aren't much fun for the humans either , but simpletons are usually satisfied to pile up cheap kills until they can shoot shiny luci bolts endlessly , walking around in a noisy armor.