Momentum + Gyroscopic effect
Steering keeps you upright in the same way a broom handle balanced on your hand is kept up by moving you hand around
Gyroscopic effects are negligable
There is a word for what you’re describing, it’s called “momentum”.
More specifically, the force vectors for gravity and forward motion combine angles in a ratio of their magnitude and the bike becomes less likely to topple than go forward (in the assumption the horizontal force is the greater of the two values), or in other words as long as it has forward momentum.