Alkyl groups act as electron donors when attached to a $\pi$ system due to hyperconjugation. When an alkyl group is attached to a $\pi$ system (such as a positively charged carbon atom in a carbocation), the electrons from the adjacent $\sigma$ bond of the alkyl group overlap with the $\pi$ system.

This overlap allows the electron density from the adjacent $\sigma$ bond to be donated into the $\pi$ system, which helps in dispersing the positive charge and stabilising the structure. Therefore, the greater the number of alkyl groups attached to the positively charged carbon atom, the greater is the hyperconjugation interaction and the overall stabilisation of the cation.