In general, when you see a block diagram, the point of it is that the blocks contain multiple or unspecified elements, but the diagram accurately represents the boundaries of, and connections between, those elements.
“Tuning” is not a single process, and in most receivers the amplification is in multiple stages interleaved with the “tuning” elements, so having a “tuning” block followed by an “amplification” block (or vice versa) would be incorrect.
“Tuning” consists of both filtering and mixing, and filtering and amplification are usually desired both before and after the mixer(s).
Another question we can ask is: given that the receiver is a complex system of many elements, why did the author of this diagram sum it up in these particular three blocks?
If I had made it that way, it would be because these three can be characterized as:
- The first block deals exclusively in RF and IF signals, and does not change the form of the signal — only its amplitude and carrier frequency.
- The second block is responsible for converting modulated IF/baseband to audio signals.
- The third block deals exclusively in audio signals.