Because the maximum stay applies to your use of the space even when nearby spaces are empty. The rule cannot change every minute with the street's occupancy.
Zona Azul organizes shared public space. The municipality sets a period so that the same space can serve different people throughout the day.
A space that is empty now may be in demand later, as shops, appointments, deliveries and schools change traffic patterns. If every driver could remain while another space looked empty, two vehicles on the same block could face different limits with no sign explaining why.
Can I pay again and stay longer?
Not always. Some areas allow renewal; others require the vehicle to leave when the maximum stay ends. Buying another period does not override a rule that prohibits continued use of the same space.
Check the operating hours, purchased period, maximum stay, renewal rule and any requirement to move to another area.
The purpose is not to punish a quiet street. It is to keep the rule predictable and prevent one person from monopolizing a scarce public asset.