A batch attack on 128 bit symmetric lowers the total cost to below 2^128 computations to successfully decrypt messages. The cost approximately follows the birthday paradox math, and thus becomes cheaper at a slightly faster than linear rate given a higher volume of ciphertexts.

A batch attack on asymmetric ECDSA like curve25519 costs MORE than 2^128, it just grows logarithmicly instead of linearly.

That's why.