Here's why:

* Klystrons are vacuum tubes specifically designed to generate and amplify microwaves. They use a process called velocity modulation, where an electron beam is accelerated and then bunched together, creating microwave oscillations.

* Traveling wave tubes (TWTs) are another type of vacuum tube amplifier that can work at microwave frequencies. They use a slow-wave structure to interact with the electron beam, allowing for amplification over a wider frequency range.

* Solid-state amplifiers, like transistors and FETs, can also amplify microwaves but are typically used for lower power applications.

So, while there are other devices that can amplify microwaves, the most common and powerful one is the klystron.