Very good question and extremely difficult to answer in simple terms. The most condensed explanation I can think of is Equilibrium Constants are numerical devices that define 'extent of reaction'. That is, it is a relative indicator of how much product is produced under specified conditions as a function of chemical reactivity. Its use to define solubility of a substance; i.e., acids in water can be misleading in that solubility is a function of solute-solvent compatibility factors whereas Ka values define the reactive ionization of a substance and not its solubility. Yes, K-values can be related to solubility, but you’ll see this more in discussions on Solubility Product Equilibrium as it relates to salt solubility (or, amount of salt that undergoes ionization in aqueous media) than in discussions comparing weak acid or weak base solubility.