There is no seal because there is no oil in the outer part of the gearbox.
The gearbox is also filled with 00 grade grease, not oil (you top-up with oil if it gets low). Which is the main reason it doesn't leak all over the place.
Here's what you see if you take the outer cover off the gearbox:
Grease could potentially leak round the kickstarter spindle where it fits in the inner cover and creep along it to reach the outside. There is an O-ring on the foot starter spindle to prevent this. This may be your issue. This picture shows both the outer and inner gearbox covers removed with the kickstarter spindle in place on the layshaft. You can see the O-ring just behind the hole where the return spring would fit.
So if you've filled it with oil, drain that and fill it with grease and your problem should go away. If not, I'd suspect that o-ring or potentially ovalling of the hole it fits in if it has seen a lot of use, which it is should become apparent if you remove the outer cover and check for play on the shaft.