As mentioned elsewhere in this thread, Carta can manage assets such as employee stock options. A company can't simply close their Carta account and declare that their corporate equity is now managed elsewhere; there's regulations that govern how everything has to be handed off.
Cap tables at most companies (including nearly all publicly traded companies) are handled internally by the Legal department or Finance department. While they may or may not use software more advanced than Excel to manage the cap table, they generally don't put all of their eggs in one basket and rely on a third-party SaaS to do this critical work for them.