Why a bulk encryption key and frequently used key needs to have a shorter lifetime?
Most symmetric encryption techniques use an $\text{IV}$. An important characteristic is that the $\{key, IV\}$ pair must never be reused.
The $\text{IV}$ is randomly generated for every use, and some cipher modes (eg: GCM) break it down into one part for the nonce, one part for the counter. eg: for a 128 bit $\text{IV}$, the nonce is 96 bits, the counter is the remaining 32 bits.
The counter usually starts at $0$ for every data and is incremented for each block. So we only care about the nonce.
Given a 96 bit nonce, the birthday paradox tells us that the probability of nonce reuse after $2^{32}$ uses is approximately $2^{-32}$. The $2^{32}$ reuse probability is recommended by NIST.
To reach $2^{32}$ uses in a week, we need to perform approximately $7000$ operations per second. This is not an outrageous number, link-level encryption can easily handle this.
So with heavy usage of a symmetric key, we want to make sure we limit the number of uses to something well below the probability of nonce reuse. Taking safety margins into account for the above example, we might pick one day as a reasonable lifetime.