Simple because it is occupied by 3 groups of methane which are highly stable and cannot b easily replaced.

Introduction
In order to understand why (CH3)3C is more stable than CH3CH2 and CH3, we need to analyze the different factors that contribute to stability in organic compounds. Stability of a compound is determined by its ability to minimize energy and maintain a lower energy state.

Electronic Factors
The stability of a compound can be influenced by the electronic factors such as electron delocalization and inductive effects.

1. Electron Delocalization: In (CH3)3C, the central carbon atom is attached to three methyl groups, which donate electron density through hyperconjugation. This leads to the delocalization of electron density across the entire molecule, resulting in increased stability.

2. Inductive Effects: In CH3CH2, the methyl group is attached to an sp3 hybridized carbon atom, which is less electronegative than the sp2 hybridized carbon in (CH3)3C. This difference in electronegativity causes the electron density to be pulled away from the methyl group in CH3CH2, making it less stable compared to (CH3)3C.

Steric Factors
Steric factors play a crucial role in determining the stability of a compound. Steric hindrance refers to the repulsion between bulky groups that can hinder the rotation of bonds and lead to higher energy states.

1. Tertiary Carbon: In (CH3)3C, the central carbon atom is tertiary, meaning it is attached to three other carbon atoms. This arrangement reduces steric hindrance, allowing for greater stability.

2. Primary Carbon: In CH3CH2, the carbon atom is primary, meaning it is attached to only one other carbon atom. This results in higher steric hindrance and lower stability compared to (CH3)3C.

3. Methyl Cation: The methyl cation (CH3+) is the least stable due to a lack of electron density. It has an empty p orbital, making it highly reactive and prone to undergo rearrangement or reaction with nucleophiles to regain stability.

Conclusion
To summarize, (CH3)3C is more stable than CH3CH2 and CH3 due to a combination of electronic and steric factors. The delocalization of electron density and lower steric hindrance in (CH3)3C contribute to its increased stability. On the other hand, CH3CH2 has higher steric hindrance and less electron density delocalization, leading to lower stability. Finally, the methyl cation (CH3+) is the least stable due to its lack of electron density and high reactivity.