Painted stairs and cracking paint

[Igloo]

Hi, my other half has painted our stairs (very old carpet was previous covering) and before doing so, applied caulk/filler to any gaps at the base of each step. But as soon as we've used the stairs, the gaps between each step has opened up.

Does anyone know of any way around this (other than putting a carpet/runner down, which we shall probably have to do!)

Many thanks