It's burst processing. You do actually need high processing speeds for short periods of time to implement network protocols like these effectively. Think cryptography, reliability, etc. The CPU isn't doing anything most of the time, but it makes a big difference if it can get the job done in 500μs instead of 5ms when there is something to do (like process a packet).
Also, higher clock speed = lower power consumption. It sounds counterintuitive, but getting the job done quickly so you can go back to low power mode sooner actually saves power, even if the instantaneous power draw while processing is higher.
Nearly all cryptography on MCUs is hardware based. It would have otherwise be completely impossible to do timing sensitive crypto work like WiFi, or BT.
The WiFi, or BT protocol stacks themselves are however almost always in software on MCUs simply because nobody would bother making a separate ASIC IP for that which will be outdated by the next standard errata.
Symmetric cryptography is often hardware based, but asymmetric crypto rarely is. The latter is commonly used for pairing/key exchanges, and would be painfully slow on a slow MCU.
Also, higher clock speed = lower power consumption. It sounds counterintuitive, but getting the job done quickly so you can go back to low power mode sooner actually saves power, even if the instantaneous power draw while processing is higher.