Then you've been playing against incompetent teams. Cap lock has a 30 second cooldown, telescroll has something similar. If you teleport out immediately as you caplock the flag (which you would need to do in order for your teleport scroll cooldown to be up again when the cap lock runs out) then the enemy player should easily be able to take back the area from the oncoming wave. Giving him control of this area allows one of 2 things.
1. Your teleport back is not timed absolutely perfectly, and even if it is, the amount of time you've allowed him to sit on the flag while you casted teleport allowed him to make the flag neuteral, blocking a caplock from you.
I don't know what the actual flag behavior is while they are locked and I can't convince the AI to help me test it (yes, I tried). What I do recall is a specific match where a Reg decided to stand next to the locked flag the entire time and I was still able to port in and lock it again.
Also, the Flag Lock is 45 seconds and the Tele Scroll cooldown is around 30 seconds. That is not an issue.
2. You make it back in time to place a second cap lock, as you fumble around with casting it and sit there while the cast timer goes off, you allow him free hits on you. Since you've just teleported your teleport cooldown is not up and so therefore now you have to fight him and he has a head start. Assuming he's not terrible, you're going to die. Putting a death penalty on you and not allowing you to get back in time to do another cap lock.
Firstly, using Erebus nullifies all your points as he can stun them with Mass Charm and run away with Bat Swarm. Other Demigods, while not quite so aptly suited, still have things they can do. Rook and Ice TB can stun the enemy while you lock, QoT has great survivability with Mulch and Bramble, Sedna is tough to kill with massive regen and Heal, and Oak can use Shield to either cap undisturbed or run away. Additionally, minion build generals are only slightly inconvenienced if the actual Demigod is busy locking the flag. (For example, Oak can pop in and quickly Surge and Penitance before starting the lock while his minion swarm does all the damage.) Also, if the timing works out, there may be allied Giants/Catas in the exchange.
And this is all assuming there is only one Demigod dealing with this on each side. You can throw some nasty Demi combinations together for an unstoppable flag re-lock. For example, Erebus starts to teleport. Oak Shields Erebus. Erebus finishes teleporting. Erebus stuns all nearby Demigods with Mass Charm (this cannot be stopped/interrupted due to Shield). Erebus locks the flag. Erebus runs/Bat Swarms away. Even if the entire enemy team is sitting on the flag, they can't stop this from happening.
And here's the best part. You don't even have to kill the defender. Its a win for you if the enemy falls back or you can manage to run away after the flag is locked. Just get in and out without getting yourself killed.