Why Do Options with Very Short Time to Expiration Generally Have Higher Gamma near ATM?

Options with very short time to expiration (short-dated) have higher Gamma near ATM because their probability of expiring ITM changes drastically with even a small movement in the underlying price. As expiration nears, the option's value is almost entirely intrinsic, and the "cliff" effect of being ITM or OTM is magnified.

This steep probability change translates directly into a high rate of change for Delta, which is Gamma.