Why is the percentage of argon in the earths atmosphere today the same as it was in the earths early atmosphere?

Argon is a noble gas and does not react with other elements in the atmosphere, so it has remained relatively constant over time. The initial percentage of argon in the Earth's atmosphere was likely determined by the gases present during the planet's formation. The lack of significant processes that remove or add argon to the atmosphere has helped to maintain its percentage over time.