Hmm because you can see the ones listed as Navagrahas (not the same as the nine planets of the solar system) in Hindu literature with your naked eyes. Here are the Navagrahas as per Hinduism:
1. Surya (Sun)
2. Soma (Chandra) (Moon)
3. Mangala (Mars)
4. Budha (Mercury)
5. Brahaspati (Jupiter)
6. Shukra (Venus)
7. Shani (Saturn)
8. Rahu (ascending / north node of the moon)
9. Ketu (descending / south node of the moon)
All of these are visible to the naked eye and back then when there was zero light pollution, keen observers could see deep into the night sky.
Also fun fact: this was not exclusive to India either. Most early civilisations had this understanding of celestial bodies long before the Indian subcontinent.
Here’s the historical / ancient observations of Mars:
The ancient Sumerians named Mars Nergal, the god of war and plague. During Sumerian times, Nergal was a minor deity of little significance, but, during later times, his main cult center was the city of Nineveh. In Mesopotamian texts, Mars is referred to as the "star of judgement of the fate of the dead". The existence of Mars as a wandering object in the night sky was also recorded by the ancient Egyptian astronomersand, by 1534 BCE, they were familiar with the retrograde motion of the planet. By the period of the Neo-Babylonian Empire, the Babylonian astronomers were making regular records of the positions of the planets and systematic observations of their behavior. For Mars, they knew that the planet made 37 synodic periods, or 42 circuits of the zodiac, every 79 years. They invented arithmetic methods for making minor corrections to the predicted positions of the planets. In Ancient Greece, the planet was known as Πυρόεις. Commonly, the Greek name for the planet now referred to as Mars, was Ares. It was the Romans who named the planet Mars, for their god of war, often represented by the sword and shield of the planet's namesake. In the fourth century BCE, Aristotle noted that Mars disappeared behind the Moon during an occultation, indicating that the planet was farther away. Ptolemy, a Greek living in Alexandria, attempted to address the problem of the orbital motion of Mars. Ptolemy's model and his collective work on astronomy was presented in the multi-volume collection later called the Almagest (from the Arabic for "greatest"), which became the authoritative treatise on Western astronomy for the next fourteen centuries. Literature from ancient China confirms that Mars was known by Chinese astronomers by no later than the fourth century BCE. In the East Asian cultures, Mars is traditionally referred to as the "fire star" based on the Wuxing system.
Thus as you can see, it’s nothing unique for ancients..
So why Galileo? Galileo has been called the father of observational astronomy, modern-era classical physics, the scientific method, and modern science.
Galileo studied speed and velocity, gravity and free fall, the principle of relativity, inertia, projectile motion and also worked in applied science and technology, describing the properties of the pendulum and "hydrostatic balances". He was one of the earliest Renaissance developers of the thermoscope and the inventor of various military compasses, and used the telescope for scientific observations of celestial objects. With an improved telescope he built, he observed the stars of the Milky Way, the phases of Venus, the four largest satellites of Jupiter, Saturn's rings, lunar craters and sunspots. He also built an early microscope.
Galileo observed the Milky Way, previously believed to be nebulous, and found it to be a multitude of stars packed so densely that they appeared from Earth to be clouds. He located many other stars too distant to be visible to the naked eye. He observed the double star Mizar in Ursa Major in 1617.
In the Starry Messenger, Galileo reported that stars appeared as mere blazes of light, essentially unaltered in appearance by the telescope, & contrasted them to planets