Bonds are statistical. We say that two atoms are “bonded” when the electrons they bring to a bond overlap in space to a degree where it takes more energy to separate those atoms than it does to keep them together. As such, they are not absolutely-defined. It turns out that, at room temperature and pressure, 1 molecule of H2O in 10^7 molecules (1:10,000,000) will break apart into hydroxide and hydronium. Let me give a graphical example of this.

There are three main features here - a low-energy left state, a high-energy intermediate state, and a medium-energy right state. Let’s arbitrarily say that the left state is H2O and the right state is H+ and OH-. When you have extra energy (high on the hill), you tend to give off energy until you reach a low point (like rolling down a hill). If you can gain energy, you can go up a hill, though. Water at room temperature and pressure has little enough thermal energy (moving around energy) to mostly stay only on the left side of the hill, but, some very small fraction of the time (10^-7, as above), it gets over that hill and rolls down the other side, separating into the two ions, even though it is more stable together.